B.SC ALLIED BOTANYCONTENTS
i. Phyllotaxy 1
ii. Types of leaves 4
iii. Inflorescencence 7
a) Racemose 8
b) Cymose 11
iv. Flower description 13
i. Bentham & Hooker’s classification 33
(i) Mimosoideae 37
(ii) Caesalpinioitae 41
(iii) Papilloionatae 43
b) Rubiaceae 48
c) Lamiaceae 50
d) Arecaceae 52
Unit – II
i. Ultra structure of a plant cell 56
ii. Mitochondria 56
iii. Chloroplast 58
iv. Endoplasmic reticulum 60
v. Nucleus 60
i. Mendel’s monohybrid cross 62
ii. Mendel’s dihybrid cross 65
1) Simple permanent tissue
i. Parenchyma 68
ii. Collenchyma 69
iii. Sclerenchyma 69
2) complex permanent tissue
i. Xylem 70
ii. Phloem 71
iii. Dicot stem 73
iv. Dicot root 75
v. Dicot leaf 77
i. Structure and development of anther 79
ii. Structure and development of Ovule 81
iii. Fertilization 85
iv. Structure and development of dicot embryo 86
i. Oedogonium 89
i. Albugo 95
i. Funaria 99
i. Lycopodium 106
i. Cycas 114
i. Osmosis 128
ii. Absorption of water 129
i. Light reaction 135
ii. Calvin cycle 140
i. Glycolysis 145
ii. Kreb’s cycle 149
i. Hydrophytes 153
ii. Xerophytes 164
UNIT – I
1.1 ARRANGEMENTS OF LEAVES
The mode of arrangement of the leaves on the stem is termed Phyllotaxy and it varies in different plants. The arrangement is said to be:
(1) Alternate when there is only one leaf at each node.
(a) When such alternate leaves are arranged to the right and left of the stem, all in one plane so that are in two rows, the arrangement is termed distichous, e.g., Polyalthia (Fig.1.1A). Annona and grasses.
Fig.1.1 Distichous arrangement of leaves (Polyalthia)
(b) When the alternate leaves are arranged spirally on the stem, the phyllotaxy is spiral. If we start from a leaf and trace round the stem, touching the bases of all the leaves on the way till the leaf lying exactly above the one we started from is reached, we might have passed through 3 or 5 leaves as the case may be and we might have traced round the stem once or twice.
(2) Opposite when leaves are arranged in pairs at each node.
(a) Such opposite leaves may all spread in one plane, i.e., to the right and to the leaf as in psidium and Quisqualis (Fig.1.1b.This arrangement corresponds to the distichous arrangement of alternate leaves, and is said to be superposd.
Fig 1.1b Superposed arrangement Fig 1.1c Decussacte arrangement
(b) Each pair may be at right angles to the one above and the one below, the arrangement being known as decussate, as in Ixora (Fig.1.1c), Calotropis and Vinca.
(3) Ternate when there are three leaves at each node arranged in a circle as in Nerium (Fig.1.1d)
Fig.1.1d Ternate arrangement (Nerium) Fig 1.1e Whorled arrangement (Allamanda)
(4) Whorled when there are more than three leaves at each node arranged in a circle as in Allamanda (Fig.1.1e).
(5) Radical when the leaves arise in a cluster from near the ground level, looking as through they start from the root directly, as in Pine apple, Agave and Aloe. In all these cases the stem is highly reduced in length and subterranean (Fig.1.1f).
1.1f Radical arrangement (Mollugo)
Significance of phyllotaxy
In order the leaves may perform their functions efficiently it is necessary that all leaves should be well exposed to sunlight and air. The various kinds of arrangement help in achieving this object and thus avoid overcrowding and shading of the leaves by one another.
Leaf Mosaic. In some like Acalypha, Begonia and Hedera, the like leaves are arranged in a mosaic. This is really “the fitting in of the leaves with one another to make the best use of light and air available.” In Acalypha (Fig.1.1g) and Begonia, the petioles of the lower leaves are longer than those of the upper ones, so that all the blades are fully exposed. In Physalis, small leaves get intercalated between larger once. In Hedera, the leaves fit together to from a more or less continuous surface,
Fig. 1.1g Leaf mosaic (Acalypha)
TYPES OF LEAVES
1.2 SIMPLE LEAF
A leaf us described as simple when there is only a single blade attached to the petiole. This blade may be undivided as in Mango and Banyan or lobed as in cotton.
1.2.a. Compound Leaf: Due to the deep incisions of the lamina, the simple leaves may become Compound They have two to many distinct blades called ‘leaflets’, each of which has a separate and distinct base attached to a common leaf stalk termed ‘rachis’. The primary leaflets are called ‘pinnae’ while the secondary ones are termed ‘pinnules’. These may be stalked or sessile. Compound leaves may either be (a) Pinnate when the leaflets are arranged along the sides of the rachis (= the midrib) or, (b) Palmate, when the leaflets are attached in one plane on the summit of the petiole.
Fig. 1.2a Types of Compound Leaves
The Pinnate compound leaves may be subdivided as follows:
1. Unipinnate, when the leaflets are borne directly on the midrib. The leaf may either be (a) paripinnate, if the number of paired leaflets is even (Tamarindus indica,) or, (b) Imparipinnate, if there is an odd terminal leaflet (Fig.1.2a).
2. Bipinnate is twice pinnate i.e., when both the primary and secondary divisions of the leaf are pinnate (Delonix regia.) (Fig.1.2a)
3. Tripinnate is thrice pinnate i.e., when primary, secondary and tertiary divisions of the leaf are pinnate(Moringa) in which sometimes the basal pinnules of the older leaf may show further divisions to form a quadripinnate compound leaf.) (Fig 1.2a)
4. Digitate-Pinnate: Here the primary division is in the palmate plan and secondary in the painnate pattaern (Mimosa pudica).
The palmate Compound leaves may be classified as follows
1. Unifoliate: Here a single leaflet is articulated at the apex of the petiole (Citrus). This actually is a modification of the pinnate kind in which all the leaflets are reduced except the terminal one which is attached to an almost obsolete petiole. (Fig. 1.2b).
Fig 1.2b Unifoliate
2. Bifoliate : Here two leaflets are articulated at the apex of the petiole (Hardwickia, Bauhinia). (Fig.1.2c)
3. Trifoliate of Ternate: There are three leaflets (Aegle marmelos) (Fig.1.2d).
4. Quardrifoliate: The compound leaf has four leaflets (Marsilea) (Fig.1.2e).
Fig. 1.2e Quardrifoliate
5. Multifoliate or Digitate: The compound leaf has more leaflets (Lupinus sativus) (Fig. 1.2f).
Fig 1.2f Multifoliate or Digitate
Decompound is a general term used for such leaves which are more than once compound and it is also applied to simple leaves which are many times dissected (Coriandrum sativum) Fig 1.2g.
1.2g Decompound Leaf
The term inflorescence refers to a cluster of flowers together with their axes, with bracts and bracteoles. The axis of the inflorescence is called as ‘peduncle’, But the axes or stalks of individual flowers are known as ‘pedicels’, The leafy structure (large or reduced) in the axil of which flowers of following branches arise are called ‘bracts’ Sometimes the bracts may form a cluster or a whorl termed ‘involucre’ (Helianthus annus). Bracteoles are structures which occur between the flower and its bract.
An inflorescence may be simple or compound, it may be small or large. The number of flowers of an inflorescence also vary from few to numerous. For example the panicles of the grasses and the capitula of the compositae are also multiflowered. Sometimes the flowers are solitary, more often they are in clusters. When solitry they may either be (i) ‘terminal’ I e., flower terminates the shoots of major or minor rank (papaver, Mangnolia) or (ii) ‘axillary’ i.e., flower formed in the axil of ordinary follage leves (Hibiscus)
TYPES OF INFLORESCENCE
Inflorescence is classified on the basis of the type of branching which may either be simple or may become compound. Accordingly an inflorescence may either be (I) Recemose or Cymose.
I. Racemose or Indeterminate: In this type the growing point cease its growth and continues forming lateral flowers which may number few to many. Hence the youngest flower is situated apically and oldest basally i.e. in acropetal succession. When the flowers from a cluster, the youngest is towards the centre i.e., the development is ‘centripetal’.
II. Cymose or Definite or determinate: In cymose inflorescence the apical growth of the primary axis and of the daughter axes are terminated by the formation of flowers. The youngest flower is formed basally i.e., the progression of blooming is basipetal or its is away from the centre i.e., the development is ‘centrifugal’.
Racemose inflorescence may be classified in to the following types:
i. Raceme: It consists of an elongated peduncle with pedicellate or stalked flowers, oldest at the base and younger flowers borne apically (Brassica campestris, Raphanus sativus) (Fig.3a).
ii. Spike: It is similar to raceme but the flowers are sessile or without stalk (Achyranthese aspera, Adhatoda vasica) Fig 1.3b. Very small spikes of secondary order are spikelets which may be grouped as units to form various arrangements like panicle, racemes or spikes (Triticum aesivum)
Fig 1.3b Spike
iii. Catkin: It is a spike or spike-like inflorescence which is usually pendulous. It may be long or short and is generally composed of apetalous, small unisexual flowers subtended by a scaly bract, the entire inflorescence falls off as a whole, Morus alba (Fig.1.3c).
iv. Spadix: It is a spike with thick, fleshy axie bearing usually minute and unisexual flowers. The entire inflorescence is generally covered over by a large enveloping, green, white or petaloid bract called a ‘Spathe’ (Colocasia, pistia,) (Fig.1.3d). Some times the axis of a spdix is branched and is covered by a stiff boat shaped spathe. Such a spadix is called as compound spadix Eg. Coconut, (See Palme family in Taxonomy section)
Fig 1.3d Spadix
v. Corymb: It is a modification of raceme, it has a very short peduncle and the lower pedicels are much elongated, thus forming a flat topped inflorescence. In matujre condition all the buds, flowers and fruits come to the same level. The opening of flowers is from periphery towards centres (cassia sp. see Leguminosae family) (Fig 1.3e)
Fig 1.3e Corymb
vi. Umbel: It is a modified receme whose penduncle is very much abbreviated and the flowers have stalks of nearly equal length so that they seem to arise from the summit of the peduncle reaching the same level and forming a sort of an umbrella. The blooming sequence is from outside towards centre. A n umbel may either be simple or compound. It is usually subtended by an involucre of green bracts (coriandrum sativum). Acymose umber occurs in Allium (Fig.1.3f,g).
Fig 1.3 f,g Umbel, Copound umbel
vii. Capitulum or Head: (See the family compositae). It is a modification of racemose inflorescence in which the peduncle ia ewsuxws And gets modified into a ‘isc’ or ‘receptacle’ which may assume various shapes- flat, concave, convex, columnar etc. Numerous sessile flowers arise over the receptacle. The opening of flowers is from periphery towards centre. A ‘head’ may either be solitary or it may be aggregated into arrangement forming panicles, racemes, corymbs, cymes (Vernonia, Helianthus (Fig.1.3h), Mimosa pudica,). A head is usually subtended by an involucre of green bracts as in sunflower. In sunflower the capitulum has two types of flowers (Ray florest and disc floret).
Fig 1.3h Capitulum
Cymose Inforescences my fall under one of the following types:
i. Uniparous Cyme of Monochasium: it is a cyme of single flower, on each axis i.e., the peduncle (main axis) bears a terminal flower and below it a lateral branch which too terminates into a single flower and so on. The terminal (central) flowers are the oldest. Commonly this inflorescence may be either of these two types viz. (z) the Helicoids cyme (Fig 1.3i) when the successive lateral axes develop only towards one side of the man axis, thus forming a spiral coil unilaterally (Hamelta patens) or (b) the Scorpioid cyme when the successive lateral axes develop alternately on both sides of the main axis. The whole inflorescence coils downwards (Heliotropium) (Fig 1.3j).
Fig 1.3 i. Monochasium Helichoid cyme Fig 1.3 j. Monochasium Scorpiod cyme
ii. Biparous cyme or Dichasium: It is a cyme where the peduncle bearing the terminal flower develops below it a pair of lateral branches which too, terminates into a flower, the central flower being the oldest. This is a simple dichasium(Unites of inflorescence in digera muricato, Salvia, coccinea) but repetition of such cymes on successive lateral branches firms a compound dichasium (Nyctanthes) (Fig.1.3k).
Fig 1.3k Dichasium
iii. Multiparous or Polychasial cyme: It is a eyme swhere more than two lateral axes, each terminating into a flower are formed below the main axis which bears a terminal and the oldest flower of the inflorescence (Calotropix procera).
Compound inflorescence: The inflorescence axes may often be repeatedly branched forming complex structures e.g. compound umbel (Fig 1.3l) Coriandum sativum). Compound capitula (Echinops. Each unit here is single-flowered), Compound spadix (Fig.1.3m) (cocos nucifera), compound spike (Triticum aestivum). Compound cormb (pyrus) Panicle (Fig 1.3n) is also a compound inflorescence, the units of which may either be racemose of cymose. The nature of the inflorescence is generally determined by the blooming progression of the flowers. The central axis in such cases is elongated, it produces branches that are themselves branched successively. It may usually be a racemose type or spicate type (many grasses) or cymose type (Amaranthus, Ancrogaphis prniculata).
Fig 1.3l Compound umbel Fig 1.3m Compound Spadix Fig.1.3n Panicle
1.4 THE FLOWER
The flower is homologous with the vegetative shoot. It is composed of a very short axis on which a few whorls of leaves called the floral leaves, are arranged.
Usually, there are four whorls of floral leaves arranged one within the other. The outer two whorls constitute the non-essential parts and the inner two the essential parts.
Parts of the flower (Fig. 1.4a)
The outermost whorl of floral leaves is collectively known as the calyx. The calyx is composed of a varying number small, green, more or less triangular structures called the sepals.
Fig 1.4 a Flower, showing parts
1. Sepal, 2. Petal, 3. Stamen, 4. Filament, 5. Anther, 6. Pistill, 7. Ovary,
8. Style, 9. Stigma
The whorl lying towards the inner side of the calyx forms the corolla. It is generally the most conspicuous part of the flower and is coloured and often scented. The corolla is composed of a number of thin, flat, delicate structures called the petals. The calyx and the corolla are together spoken of as the perianth.
The first of essential organs lies immediately next to the corolla on its inner side and is known as the androecium. The androecium represents the male reproductive structures of the flower and is composed of number stamens. Each stamen has a long slender stalk, the filament, at the upper end of which is attached an elongated, usually bilobed, box- like structure known as the anther. Inside the anther is present a very fine, golden yellow powder called the pollen or the pollen grains.
The second set of essential organs is known as the gynoecium or the pistil and is situated right in the centre of the flower. The pistil consists of a small, hollow box known as the ovary, the tip of which extends as an elongated, slender structure called the style.
Fig 1.4b Flower – Vertical Section
1. Sepal, 2. Petal, 3. Stamen, 4. Filament, 5. Anther, 6. Pistill, 7. Ovary,
8. Style, 9. Stigma
The specialized tip of the style is called the stigma. Inside the ovary, are found a number of ovoid structures known as the ovules (Fig. 1.4b). It is the ovule which later develops into a seed. So the ovule is the immature seed. The gynoeeium represents the female reproductive structure of the flower.
The distribution of sex in flowers
1. Flowers having both the androecium and the gynoecium are said to be bisexual or hermaphrodite and are denoted by the symbol ? e.g., Thespesia.
2. Flowers in which only one set of essential organs is present, are said to be unisexual. (a) If they have only the androecium, they are staminate or male flowerers and are noted by the symbol ?;
b) if the flowers have only gynoecium, they are said to be pistillate or female flowers and are denoted by the symbol ?, e.g. Cephalandra, cucurbita and castor.
3. When both the staminate and the pistillate flowers occurs on the same plant, they are said to be monoecious as in Cucurbita, and Ricinuus and dioecious when they are on different plants, so that there are distinct male and female plants, as in palmyra and data palms.
4. When unisexual and bisexual flowers occur on the same plant as in Feronia and mango, they are described as polygamous.
When all the four worlds the calyx, corolla, androecium and gynoecium are present in a flower, the flower is said to be complete, e.g., Tribulus, Hibiscus.
When one or more of the whorls is absent the flower is incomplete, e.g., Clematis, Sterculia.
Symmetry in Flowers
1. When the parts of each whorl are uniform in size and shape, the flower is regular as in Tribulus; and when the parts of a whorl are unequal in size, as in Crotalaria and Ocimum the flower is said to be irregular.
2. A flower, which is capable of being divided into two exactly similar halves along any vertical plane, is said to be actinomorphic, e.g., Tribulus, and Ipomoea. Complete regular flower are always actinomorphic.
3. A flower which is can be divided into two similar halves only along one particular plane, is said to be zygomorphic. Such flowers are irregular, e.g, Crotalaria, Leucas, Salvia.
Actionmorphic and zygomorphic flowers are said to be symmetrical.
4. If a flower cannot be divided into two similar parts in any plane as in Canna, it is asymmetrical.
The calyx forms the outermost set of floral leaves. Its component parts are the sepals. The sepals are usually green in colour.
Fig 1.4 c (a) Leafy Sepal (Mussaenda) Fig 1.4 d (b) Calyx
In some plants like Caesalpinia, Fuchsia, and certain Salvias, the Calyx, instead of being green, is brightly coloured. Such a calyx is said to be petaloid. In Mussaenda, one of the sepals of the calyx grows out as a large whitish or yellowish leaf-like structure Fig 1.4c (a).
The sepals may be either free from each other when the calyx is said to be polysepalous or they may be united, when the calyx is known as gamosepalous [Fig. 1.4c (b), 1.4d (b)]. At times the calyx gets modified into a number of fine hairly structures called Pappus [Fig. 1.4 d (b)]. This is the case in Tridax and Vernonia. In the common balsam one of the sepals is prolonged downwards into a tubular structure called the spur. The calyx is then said to be spurred [Fig. 1.4 d (b)].
The corolla constitutes the second set of floral leaves. It is situated between the calyx and the androecium and is usually composed of a single whorl of petals. As contrasted with the sepals, the petals are generally thin, delicate and highly coloured, thus making the flowers conspicuous, showy and attractive, many cases the petals are also scented.
In some flowers like Annona and Polyalthia, the petals are thick, succulent and green in colour. Such petals are said to be sepaloid.
When both the calyx and the corolla are present in a flower, it is described as being dichlamydeous. But when only one whorl of perianth leaves is present, the flower is said to be monochlamydeous. In all such cases, it is the corolla which is considered to be absent, and as such the flowers is also described as being apetalous.
Like the sepals, the petals may be either free from one another or they may be united either wholly or partially. In the formed case the corolla is a said to be polypetalous and in the latter, gamopetalous. In the polypetalous corolla, the petals are often provided with a small, narrow, stalk- like lower portion called the claw and an upper expanded portion, the limb. The petals are said to be clawed. In possessing this stalk or claw the petals resemble the foliage leaves more than the sepals do, as the sepals are always sessile.
In the gamopetalous corolla, the lower portion is known as the “tube” and the upper portion the “limb”. The upper portion is often lobed, the number of lobes indicating the number of petals in the corolla.
When the petals composing the corolla are all of the same size and shape and are arranged in a symmetrical manner, the corolla is regular. When the petals vary in size and shape and are not symmetrically arranged, the corolla is irregular.
Depending on these characters, four types of corolla are recognized, and under each type there are several varieties.
I. Polypetalous regular corolla
1. The cruciform corolla: This corolla is made up of four clawed petals arranged in the form of a cross, e.g., mustard, radish (Fig. 1.4 e1).
Fig 1.4 e Polypetalous Corolla
2. The caryophyllaceous corolla: Here, there are five petals with long claws, and the limbs spread out at right angles to the claws, e.g., Dianthus (Fig. 1.4e2-2).
3. The rosaceous corolla: This is compound of fie spreading; petals with very short claws, e.g., Rose, Rubus, Sida (Fig. 1.4e-3).
II. Polypetalous irregular corolla
There are many kinds of polypetalous, irregular corolla with no special names. There is one, however, which is widespread in occurrence, and is known as the papilionaceous corolla because of its fancied resemblance to a butterfly (Fig.1.4e-4).
III. Gamopetalous regular corolla
1. The tubular corolla: Where it is in the form of a cylindrical tube as in the central florets of sunflower (Fig. 1.4f-1).
Fig 1.4 f Gamopetalous Corolla
2. The companulate or bell-shaped corolla: When the corolla is rounded at the base and spreading above like a bell, e.g, Physalis, pumpkin
3. The infundibuliform or funnel-shaped corolla: Here the corolla gradually tapers down to a narrow base as in Tobacco, Datura and Ipomoea (Fig. 1.4f-3).
4. The hypocrateriform or salver-shaped corolla: Consists of a long narrow tube and a limb placed at right angles to it as in Vinca (Fig. 1.4f-4).
5. The rotate or wheel- shaped corolla: This is just like the salver-shaped corolla, but has a short tube as in Nyctanthes and Solanum (Fig. 1.4f-5).
IV. Gamopetalous irregular corolla
1. The bilabiate or two- lipped corolla: Here the limb is cut up into two portions or lips, an upper narrow one formed by two petals and a lower, broad, spreading one formed by the fusion of three petals. The throat, or orifice of the tube, is always open, e.g., Ocimum, Leucas (Fig. 1.4f-6).
2. The personate or masked corolla: This resembles the bilabiate corolla in having two lips, but the lips come so near each other that the throat is closed, e.g., Calceolaria.
3. The ligulate or strap- shaped corolla: Here the corolla consists of a lower, short, tubular portion and an upper, long, narrow, flat, strap-like portion at the end of which are five small lobes indicating the number of component petals, e.g., outer florets of sunflower (Fig. 1.4f-7).
The small, tender leaves are folded and corollary arranged in a compact manner in various ways in the vegetative bud. Similarly, the component pars of the calyx and the corolla are arranged in different ways in a compact manner in the flower bud. The arrangement of the sepals or petals in relation to one another is known as aestivation. The following types are more commonly met with in flowers:-
1. Valvate: When the petals or the sepals just touch one another without overlapping as in Annona and the central tubular florets of Helianthus (Fig. 1.4g-1).
2. Imbricate: when the petals or the sepals are found overlapping one another in an irregular manner (Fig. 1.4g-2) as in Caesalpinia or Bean. The imbricate aestivation of the papilionaceous corolla is termed vexillary aestivation.
Fig 1.4 g Aestivation
3. Controled or twisted: When the parts overlap one another in a regular manner, so that each petal or sepal overlaps on one side and is overlapped on the other side, the result is that the bud gets a twisted appearance, e.g., Ixora, Thespesia, Hibiscus (Fig. 1.4g-3).
4. Induplicate: When the parts are folded inwards as in the corolla of Datura (Fig. 1.4g-4).
5. Induplicate- convolute: This type is a combination (3) and (4) and is met with in the flowers of convolvulus, Evolvulus and Ipomoea.
The stamens constitute the third set of floral organs, and the first set of essential organs. Each stamen consists of a filament bearing at its tip and anther. The anther exhibits two distinct lobes externally. When the anther is young, each lobe contains two small cavities or chambers. As the anther develops, the two cavities in each lobe become bigger in size and finally run into each other, so that in the fully developed anther there are two big caviti8es, one inside each lobe (Fig. 1.4h). These cavities are filled with pollen grains. The two anther lobes are connected to each other and to the filament by a whitish tissue called the connective.
Fig 1.4 h Anther
The filament varies in size, shape and colour in various plants.
In some cases, it may be broad and expanded, with the anther attached to one side, when it is said to be petaloid, e.g., water-lily and Canna (Fig. 1.4i-1). In most cases the filament is simple. But at times, as in Castor, the filament is branched and at the end of each branch there is an anther (Fig. 1.4i-2).
Fig 1.4 i
Usually the filament is white in colour. But at times it is coloured like the corolla as in Poinciana and Acaia.
The filament may be either erect or curved. When it is curved inwards, it is said to be incurved, and recurved when curved outwards (Fig. 1.4j).
Fig 1.4 j Incurved Fig 1.4 k Inserted
(1) And Recurved (2) Stamens (1) And Exserted (2) Stamens
In some flowers the filaments are shorter than the corolla tube, so that the stamens are not seen outside the corolla. Such stamens are said to be inserted or included. When, however, the filaments are long, so that the statements project beyond the corolla, they are said to be exserted or protruding (Fig. 1.4k).
The anther usually consists of two longitudinal lobes placed on either side of the upper part of the filament which forms the connective. The connective is attached to the back of the anther. The opposite side of the anther is the face, and it always presents a grooved appearance. When in a flower, the anthers are so situated that their faces are turned towards the centre of the flower, the anthers are said to be introrse, e.g., Water-lily. But when the face of the anther is turned outwards, the anther is described as extrorse, e.g., Lily.
Attachment of the anther
The anthers are attached to the filaments in three distinct ways
(Fig. 1.4 l). (1) In some cases as in Water-lily, or Champaka, the filament is attached to the back of the anther, throughout its length and the anther is said to be dorsifixed or adnate. (2) In others like Solanum and Cassia, the extreme tip of the filament is attached to the base of the anther, such an anther is said to be basifixed. (3) In Grasses, and Poinciana, the filament is attached to the middle of the anther so that the anther swings freely on the filament. This kind of anther is described as versatile.
Fig 1.4 l Attachment of Anthers
Dehiscence of the anther
The pollen grains which are developed inside the cavities or pollen sacs of the anther are important male reproductive structures, and in order that they may be useful, they should be set free from inside the anther. For this purpose, when the pollen grains are ripe, the anther breaks open. This opening of the anther for liberating the pollen grains is known as the dehiscence of the anther (Fig. 1.4 m).
Fig 1.4 m Dehicence of Anthers
1. The most common method of dehiscence is the opening of each lobe of the anther, by a longitudinal slit. The slit widens and the pollen is liberated. This is called longitudinal dehiscence.
2. In Some cases like Cassia and Solanum, each lob e of the anther opens at the tip by means of a small hole or pore. This is known as apical or porous dehiscence.
3. In Cinnamomum and barberry, the anther dehisces in a peculiar manner. At the top of each lobe of the anther, the wall opens like a trap door or shutter and through the opening thus produced, the pollen grains escape. This kind of dehiscence is called valvular dehiscence.
4. In a few cases, the anther splits open transversely or in a horizontal manner. This transverse dehiscence is seen in Lemna and Leucas.
Generally the connective is narrow and the two lobes of the anther are close to each other. But at times, the connective broadens out and the anther lobes get separated from each other as in Tecoma when the anthers are said to be divergent. In Salvia, the connective becomes elongated and carries an anther lobe at one end, while the other end is barren. Sometimes, the connective gets prolonged beyond the top of the anther, either into a broad, membranous structure as in Annona, in to a small rounded structure as in Adenanthera, into a spinose projection as in Trichodesma or into a long feathery structure as in Nerium (Fig. 1.4n).
Fig 1.4 n Anthres showing modifications of the connective
Arrangement of stamens
The stamens may be arranged spirally as in Annona, Magnolia, etc., or they may be arranged in whorls. In some flowers they are many, arranged in many concentric whorls as in guava; or, they may be in two whorls (twice as many as petals). When they are in two whorls, they may be arranged in either of the following ways: the outer whorl alternates with petals and the inner whorl is opposite the petals (diplostemonous) or, the outer whorl in antipetalous and the inner whorl is alternating with the petals (obdiplostemonous).
Insertion of the stamens
The stamens may start from the petals, when they are said to be epipetalous. In all gamopetalous corollas, the stamens are epipetalous. When the stamens are not attached to other floral parts they are said to be free.
Union of stamens
The stamens may be all free or united in various ways:
1. When the filaments of all the stamens are fused together to form a tubular structure, enclosing the pistil, they are said to be monadelphous, e.g., Crotalaria, Thespesia (Fig. 1.4o-1).
Fig. 1.4 o Stamens
2. When the filaments are untied into two bundles, the stamens are said to be diadelphous. In Erythrina and Clitoria, there are ten stamens of which mine are united and the tenth is free. The tube formed by the filaments instead of being complete, has a narrow longitudinal slit on one side and the free stamen is placed in this slit (Fig. 1.4 o-2).
3. In Pomelo and Bombax, there are numerous stamens and their filaments are united into a number of groups or bundles. Such stamens are polyadelphous (Fig. 1.4 o-3).
4. When the filaments are all free, but the anthers are all united so as to form a tube, the anthers are said to be syngenesious e.g., sunflower
(Fig. 1.4 o-4).
Length of stamens
All the stamens of a flower may be of the same length, or of different lengths. In mustard and radish, there are six stamens, of which four are long, and two short. The long stamens are all of the same height. So also the short ones. The stamens are described as tetradynamous (Fig. 1.4 p-1).
Fig 1.4 p Stamens
In Leucas, there are two pairs of stamens, one pair long and one pair short. The androecium in this case is said to be didynamous (Fig. 1.4 p-2).
Usually, the pollen grains are in the form of a loose powder, lying free inside the pollen sacs. But, in a few cases, all the pollen in a sac may be gathered together so as to form a single pollen mass, known as a pollinium. Pollinia are seen in Calotropis and Vanda.
The torus is that part of the flower stalk which is above the point of attachment of the calyx and so is within the flower. It is also known as the thalamus or the receptacle. It is made up f four nodes, with the internodal portions between them highly reduced. The four sets of floral parts are arranged on these four nodes.
In most plants the torus presents nothing remarkable, and is just the ordinary tip of the flower stalk. In some plants, however it is modified in various ways, thus causing considerable changers in the form of the flower. In some cases as in Michelia (Fig.1.4 q-1), the torus is elongated and cylindrical, while in Annona it is elongated and conical. This form of elongated torus is usually found in acyclic and hemicyclic flowers.
Fig 1.4 q Modifications of Torus
In Nelumbium (lotus), the torus is in the form of a large expanded structure with a flat top having numerous small pits. In these pits are sunk the numerous separate carpels (Fig. 1.4q-2). In the wild rose, it is the form of a deep cup (Fig. 1.4q-3) with the calyx, corolla and the stamens placed at the rim. Occasionally, the torus becomes flattened out into a thick, cushion- like structure called the disc, between the stamens and the pistil. Such a disc is found in the flowers of orange and pomelo (Fig. 1.4q -4).
When the thalamus presents no modification, it is just a small convex protuberance above the calyx, bearing at its summit the pistil. So the pistil is naturally at a higher level than the other parts of the flower. The ovary is then described as being superior. Since the other parts are below the pistil, the flower is said to be hypogymous, e.g., Thespesia, orange (Fig. 1.4 r-1).
Fig 1.4 r Modifications of the Torus
Occasionally, due to irregular growth, the torus becomes hollowed out into a cup-shaped structure. The pistil is situated inside the cup and the other floral parts are arranged on the edge of the cup, surrounding the pistil. The ovary is now said to be half-inferior and the flower, perigynous (Fig. 1.4r-2), e.g., Caesalpinia.
In many plants, the torus gets completely hollowed out and fuses with the ovary, so that the calyx, corolla and the stamens appear to start from the top of the ovary. Since they are all above the ovary, the flower is described as epigynous (Fig. 1.4r-3). The ovary is now said to be inferior, e.g., Psidium, Cephalandra.
The Gynoecium or Pistil constitutes the fourth set of floral organs and the second set of essential organs It consists of a basal, small, box-like structure, the ovary, from the top of which grows out a slender, elongated style. The tip of the style is modified into a specialized portion known as the stigma. Inside the ovary are found the ovules, attached to thickened ridges called placentas.
Like all the other floral parts the Gynoecium too is considered to have been derived from specialized structures called the carpellary leaves or carpels. If a leaf bearing ovules at the margin gets folded along its midrib and fuses at its edges, with the margins slightly infolded, a simple type of pistil will result. The tip of the leaf forms the style (Fig. 1.4s. Such a pistil, because it is formed by a single carpellary leaf, is known as a monocarpellary pistil, e.g., bean, pea, Crotalaria.
Fig 1.4 s Carpel – Conception
1. Carpellry leaf, 2. Monocarpellary pistil, 3. Syncarpous pistil formed by closed carpels, 4. Syncarpous pistil formed by open carpels
In most cases, however, the pistil is composed of more than one carpel, so that the pistil is said to be compound. In a compound pistil, the carpels may be free as in Annona and Michelia, when the pistil is said to be apocarpous. In an apocarpous pistil, there are numerous ovaries, each with its own style and stigma. If the carpels are fused together, the pistil is syncarpous.
When the ovary has a single cavity, it is described as unilocular or one- celled and when there are more cavities than one, as bilocular, trilocular, etc. Similarly, the number of carpels forming a syncarpous pistil is indicated by suitable prefixes as bicarpellary, tricarpellary and so on.
The fusion of the carpels
In the syncarpous pistil, the carpels may be fused in different says:
1) The carpels may be fused by their entire lengths, i.e., the ovaries, the styles and stigmas may all fuse together as in Thespesia (Fig. 1.4t-1).
2) The carpels may fuse only in the lower portions so that the ovaries are united while the styles are free as in Dianthus (Fig. 1.4t -2).
Fig 1.4 t Fusion of Carpels
3) The ovaries and the styles may be united so that the stigmas alone are free as in shoe-flower and Sida (Fig. 1.4t-3).
4) The ovaries may be free while the styles and stigmas are untied as in Vinca (Fig. 1.4t-4).
5) The styles and the ovaries may be free while the stigmas alone are untied as in Calotropis (Fig. 1.4t-5).
The ovules are attached to thickened ridges or projections inside the cavity of the ovary. These ridges are known as placentas, and the term placentation is used to denote the manner in which they are distributed:
1) Marginal placentation: In the simple, monocarpellary unilocular pistil, there is only a single placenta formed by the fused edges of the carpellary leaf and situated on the ventral suture. Such a placentation is said to be marginal, e.g., bean, Crotalaria (Fig. 1.4 u-1).
2) Axile placentation: In syncarpous ovaries formed by the fusion of closed carpels, the placentas are found at the inner angles of the cells. The number of placentas is the same as the number of carpels forming the ovary, e.g., shoe-flower, Thespesia, Agava, Canna (Fig. 1.4 u -2).
Fig 1.4 u Placentation
1. Marginal, 2. Axile, 3. Parietal, 4 & 5. Free central 6. Basal, 7. superficial
3) Parietal placentation: In syncarpous ovaries formed by the union of open carpels, the placentas are found along the inner wall of the ovary which is unilocular. Here too there are as many placentas as the number of carpels, e.g., Argemone, Passiflora (Fig. 1.4 u -3).
4) Free central placentation: If in a syncarpous ovary formed by closed carpels, the cross-walls separating the cells either break down or fail to develop, the placentas will be found forming a pillar-like strcutre, standing in the middle of the single-celled ovary (Fig. 1.4 u-4 and
1.4 u 5). Such a placentation is said to be free central, e.g., Dianthus, Portulaca, Santalum.
5) Basal placentation: when there is only one ovule, and that is attached to the base of the ovary which is unilocular, as in Helianthus and Polygonum, the placentation is said to be basal (Fig. 1.4 u -6).
6) Superficial placentation: Here the ovary is many-celled and the ovules are attached to the cross- walls between the cells, e.g., water-lily
(Fig. 1.4 u-7).
The style is the prolongation of the tip of the carpellary leaf. It is an elongated tubular structure, generally tapering from below upwards. Its outer surface may be either smooth, or hairy. Usually, the style is found right on the summit of the ovary and is said to be apical. At times it is displaced towards one side of the ovary and is than described as lateral. In Leucas, Ocimum and Heliotropium, the pistil is syncarpous and the carpels are fused only at the base. The style which are lateral are all united together, so that the compound style is seen starting right from the base of the ovary and coming out between the lobes of the ovary. Such a style is said to be gynobasic (Fig. 1.4 v). The style may be either simple or branched as in Euphorbia. At times it becomes broadened out and flattened as in Canna. It is then said to be petaloid.
Fig 1.4 v Gynobasic Style
Usually the stigma is separated from the ovary by the style. But in some cases as in Poppy, the style is absent and the stigma is found directly is the top of the ovary. Such a stigma is termed sessile (Fig. 1.4w). The stigma may be either simple as in the bean, pea and Crotalaria or branched. The branches (also called at times stylar branches) may be equal in number to the number of carpels composing the ovary as in Hibiscus and Sida or twice the number as in Pavonia. In all cases the stigma is slightly sticky to the touch. In some it is hairy or feathery, so as to receive the pollen grains.
Fig 1.4 w Sessile Stigma (Poppy)
Ovules are small oval structures found inside the ovary. They are attached to the placentas by short stalks. Each ovule is an immature seed.
1.5 BENTHAM AND HOOKER’S SYSTEM OF CLASSIFICATION
Below is given a synopsis of Bentham and Hooker’s system:-
A. Polypetalac (Corollas of separate petals).
Series I – Thalamiflorae (Stamens hypogynous and usually, many,
no disc present)
Cohorts: Ranales, Parietales, Polygalineae, Caryophylineae, Guttiferales and Malvales.
Series II – Disciflorae (Staments hypogynous, discpresent). Cohorts:
Geraniales, Olacales, Celastrales, Sapindales.
Series III – Calyciflorae (Stamens perigynous or epigynous, Ovary
Cohorts: Rosales, Myrtales, Passiflorales, Ficoidales and Umbellales.
B. Gamopetalae (Corolla of partially or completely connate petals.
Series I – Inferae (Ovary inferior). Cohorts: Rubiales, Asterales
Series II – Heteromerac (Ovary superior, androceium 1, or 2 series,
carpels mostly more than 2).
Cohorts: Ericales , Primulales and Ebenales.
Series III – Bicarpellate (Ovary superior, androcium of 1 series,
Cohorts: Gentianales, Polemoniales, Personales, Lamiales.
C. Monochlamydeaee (Flowers Apetalous).
Series I – Curvembryae (Embryo coiled, ovule mostly 1)
Nyctagineae, Chenopodiaceae, Amarantaceae, etc.
Series II – Multivoulatae aquaticae (Several seeded, immersed
aquatics). Prodostemmaceae, etc
Series III – Multiovulatae terrestres. Nepenthaceae, Aristolochiaceae,
Series IV – Micrembryae (embryo minute in endosperm). Piperaceae,
Series V – Daphnales (Ovary unicarpellate, uniovulate). Laurinae,
Series VI – Achlamydosporae (Ovary usually inferior, unilocular,
ovules 1-3). Santalaceae, Loranthaceae, etc.
Series VII – Unisexuales (Flowers unisexual). Euphorbiaceae,
Series VIII – Orders anomaly (of uncertain relationships nearer to VII
than to anything else.) Ceratophyllaceae, etc.
Gnetaceae, Coniferae, Cycadaceae
Series I – Microspermae (Ovary inferior, seeds minute).
Orchidaceae, Hydrocharitaceae, etc.
Series II – Epigynae (Ovary usually inferior, seeds generally large).
Scitamineae, Amaryllidae, Irideae, etc.
Series III – Coronarieae (Ovary superior, perianth coloured).
Series IV – Calycineae (Ovary superior, perianth greenish).
Juncaceae, Palmae, etc.
Series V – Nudiflorae (Perianth mostly none, seed albuminous).
Pandanaceae, Aroideae, etc.
Series VI – Apocarpae (Pistils more than one and distinct).
Alismaceae, Naidaeace etc.
Series VII – Glumaceae (Perianth reduced, Scaly bract present,
conspicuous). Cyperaceae, Gramineae.
Merits and demerits of Bentham and Hooker’s system
Criticism leveled against Bentham and Hooker’s system arise from the basic fact that this system has not given due weight age to facts of evolution. But if we mind that at least at the time of the publication of their system, these two authors were not convinced evolutionist, we would be inclined not to be too critical.
1. Bentham and Hooker’s system of classification is a natural system of classification. It does not bring out the phylogenetic relations of the various groups of plants: neither was any such claim made by the authors. In fact, it was put forward at a time when the concept of evolution did not yet firmly root itself in to minds of men. However, it gainsaid, that as a key it serves very well.
2. The position of gymnosperms is an anomaly which could have been easily avoided. The Gymnoperms are, without doubt, a more primitive group of plants than Angiosperms. As such, their interpolation between dicotyledons and monocotyledons is an error.
3. Whether Monochlamydeae should be regarded as a group derived by reduction f other dicotyledonous flowers is a point of debate. Engler, Hutchinson and others regard that group as primitive, clubbing them together with polypetalae, under Archichlamydae. In all possibility, while some of the monochlamydeous flowers represent an advanced condition by way of being reduced dichalamydeous flowers, others are surely primitive. Thus an anomalous position where advanced and primitive families are clubbed together arises.
4. Which are a more advanced group of plants, dicotyledons or monocotyledons? This is a question which has been baffling plant evolutions and recent research work seems to justify the view of Bentham and Hooker that decotyledons are an older group than monocotyledons, though Engler thinks otherwise.
5. The subdivision of the monocotyledons is made on the basis of the relative position of the ovary and characters of perianth. This has resulted in the anomalous situation of having to club Irideae and Amaryllideae, closely related to Liliaceae, along with Scitamiane and Bromeltaceae.
In spite of all these defects, the great merit of this system is that it is a very valuable guide to the identification of the various genera of seed-plants, inasmuch as it represents a very careful comparative examination of seep-plants by two very eminent taxonomists.
Class :Dicotyledons Sub-class : Polypetalae
Cohort:Rosales Series : Calyciflorae
This is the second largest family of flowering plants comprising about 600 genera and 12,000 species. Also it ranks second in the economic importance & cosmopolitan in distribution. Leguminosae exhibits a diversity in both vegetative and floral characters. Therefore Bentham and Hooker sub-divided the family Leguminosae into three sub families. Of the three sub-amilies Mimosideae is the most primitive, papilionacaae is the most advanced. So the order of arrangement of sub-families is as follows. (1) Mimosoideae (2) Caesalpinoideae and (3) Papilonatae.
1.6 a. Mimosoideae
Cohort: Rosales Series: Calyciflorae
Distribution : Members of Mimosodieae are distributed in tropical and subtropical regions.
Habit: Mostly trees Pithecalobium, Albizzia . Eg; Enterolobium, or Shrubs (Prosopis.). Rarely herbs Mimosa pudica. Rarely a xerophyte as in Acacia melanoxylon where the petioles become flattened and green forming the phyllodes. Hydrophyte as in Neptunia. Sometimes a huge liane as in Entada gigas which climb upon the support by means of the tendril. Usually the plants are covered with spines.
Leaves: Usually a bipinnately compound leaf. Rarely pinnately compound. Pulvinate, alternate and stipulate. In xero-phytic members Acacia melanoxylan the leaflets are very much reduced and the carbon assimilation is done by the flattened, green, leaf like phyllodes. Usually the leaf-lets show sleeping and sensitive movement due to change of turgidity of the cells present in the pulvinus. Usually the stipules are modified into long or short spines.
Inflorescence: It is a recemose type namely a spike, or condensed spike, Rarely a globose head as in Parkia (”Badminton ball” tree).
Flowers: Small , usually bisexual, bracteate, bracteolate, penatmerous except gynoecium, Actinomorphic, and perigynous, Rarely tetramerous as in Mimosa pudica.
Calyx: Sepals 4 or 5 gamosepalous, Valvate aestivation. The odd sepal is always anterior when there are 5 sepals.
Corolla: It is not attractive, Petals 4 or 5 united,(exception) among polypetalae) rarely free. Aestivation is valvate. The old petal is always posterior in position. The corolla is regular.
Fig.1.6 a. Mimosa pudica
Androecium: It s the attractive part of the flower, Number of stamens equal to or twice the number of petals. Rarely numerous. They are free. Monadelphous in Pithecalobium, Albizzia. Stamens are exerted and filaments are long and brightly coloured. Antheres dithecous. A 4 Free in Mimosa, a 10 Free in prosopis, A(10) Monadelphous in Parkia, A Free in Acacid, A(?) Monadelphous as in Pithecalobium.
Gynoecium: Monocarpellary. Ovary half superior, Uniloculr, numerious ovules on marginal placentation. Style long and stigma minute and simple.
Fruit: Is the legume or lomenturm. Seeds are usually non-endospermous. Rarely arillate as in Pithecalobium. (The aril is the ousgrowth of the chalazal part of the ovule).
1. Acacia Arabica: (Tam: Karuvelamaram) A type of gum is collected by making injury on the stem. The gum is commercially and medicinally useful. The bark is useful-from the bark tooth powder is prepared. The wood is useful for making agricultural implements and in the construction of country boats.
2. Acacia concinna: (Soal nut) Dried pods are powdered and used for removing oil. (Tam: Cheyakkai)
3. Albizzia procera: timber is useful for making agriculture implements and tea boxes.
4. Pithecalobium ducie: (Tam: koddukaipuli tree. The seeds are covered with white or pink aril. The aril is edible.
5. Mimosa Pudica: ‘touch me not’ plant) Leaves show sensitive movement.
6. Prosopis specigera: Small tree, stipular spines, cultivated as life fences. Wood used as fuel.
7. Enterolobium saman: (Rain tree) Leaves show sleeping movement during night. Planted as tree.
1.6 b. Caesalpinioidaae (The cassia family)
Cohort: Rosales Series: Calyciflorae
Distribution: Mostly tropical in distribution.
Habit: Mostly trees, Delonix, Tamarindus, Parkinsonia, etc, Rarely shrubs-Cassia auriculata. Herbs are very rare. Sometimes a huge liane as in Bauhinia which climbs upon the support by means of auxiliary bud hook.
Leaves: Pinnately compound in Tamarindus, Bipinnately compound in caesalpinia, delonix, etc. Bifoliately compound in Bauhinia. Pulvinate, stipulate and alternate. In the xerophytic member like Parkinsona the leaf-lets are very much reduced and photosynthesis is done by the flattend green secondary rechis called the phyllodes. The primry rachis is reduced to a spinous structure.
Fig.1.6 b Cassia occidentilis
Inflorescence: It is a raceme, corymb or a panicle
Flowers: Bixexual, irregular, pedicellate, Bracteate, pentamerious and perigynous.
Calyx: Sepal 4-5 polysepalous and irregular. The old sepal is usually larger hooded and anterior, in position. In the case of Tamarindus the posterior two sepals are united. So the calyz has only 4 sepals. Rarely the sepals are coloured yellow as in caesap\lpinia pulcherrima. Imbricate aestivation.
Corolla: Petals 4 to 5, polypetalous irregular. Rarely flowers are apetalous (Petals are absent) in Saraca, of the five petals, one is different in shape and clawed. This clawed petal is posterior and innermost in bud. Sometimes 3 petals are present and the remaining two petals are suppressed or representd by bristles as in Tamarinuds. Aestivation ascendinly imbricate.
Andoecium: 10 Stamens, Diplostemonous, Filaments are free and unequal. Rarely there are only 3 fertile stamens and monadelphous, (Eg. Tamarindus). The number of fertiles stamens very from species to species. In some species of Cassia some of the posterior stamens are sterile. Anthers are dithecous. Basifixed or versatile, dehiscing by longitudinal slits or by pores.
Gynoecium: Monocarpellary, Unilocular, one or two vertical row of ovules in marginal placentation, ovary half-interior. Style simple and stigma capitate. The ovary has a small stalk (gynophore) at the base.
Fruit: Legume, sometimes samara as in Pterolobium. Seeds with or without endosperm.
Popular examples and economic importance.
1) Casssia auriculata; C tora, C, tanceolata (Tam-tirunelveli senna in these plants bark, fruits seeds and leaves are used for tanning (Indian Laburnum – Tam: Sarakkondrai).
2) Cassia fistula. C.Lavanica. C.Stamia, Delonix irgia Caesalpinid pulcherrima, Caesalpinia coriaria-Divi-divi plant). Bauhinia purpuria, Saraca Indica are grown in the garden for ornamentation and as avenue tree.
3) Tamarindus indica: (Tam: puli) Tree, cultivated for the fruits, Pulpy mesocarp forms the tamarind of commerce. It is used for cooking. Timber is also useful.
4) Haematroxylon campechianum: A stain ‘haematoxlyin” is obtained from the heart wood. It is used in the laboratory as a nuclear staining material.
5) Parkinsonia aucieata: Xerophytic tree, Leaves bipinnately compound The primary rachis is modified into a spine and the secondary rachii are modified into long, narrow, flat, green, phyllodes. Leaf-lets are very much reduced.
1.6 (c) Papillionatae
Cohort : Rosales Series: Calyciflorae
1. Shrubs or trees 2. Compound leaves 3. Corymb or panicle inflorescence
4. Irregular perigaynous flowers 5. Stamens 10 free. 6. Hall superior ovary
7. Fruit legume.
Habit: Mostly herbs Crotalario, Arachis, Tephrosia; Shrubs Crotalalaria sp; Trees- Pongamia, dalbergia, Twiner-Clitoria, Dolichos, tendril climber-pisum sativum Lathyrus. A xerophyte-Ulex, A hydrophyte- Aeschynomene aspera (pith-plant, Tam: Netti) Members of this family are remarkable for their nitrogen flaxation in the root nodules.
Leaves: Simple or pinnately or palmately-ompound; alternate; stipulate; Leaf-base is pulvinate. The leaf-lets are sometime stipellate. The number of leaflets in the compound leaf varies. Simple leaf in Crotalaria species. Trifoliately compound Dolichos, Erythrina, pinnately compound with many leaf-lets in Pongamia. Terminal lef lets modified into tendrils as in Pisum & Lathyrus.
Fig 1.6 C. Clitoria ternatea
In the tendril climber like pisum sativum the terminal leaf-letsare modified into tendrils. In Lathyrus the entire leaf is,modified into a tendril and the carbon assimilation is done by the large leaf-like stipules, In the xerophytic members like Utex europeaus, Cytisus etc, the leaves are very much reduced into spines or scales respectively and the stem is photosynthetic (phylloclade).
Inflorescence: Always Racemose. It is a raceme or spike. Rarely condenced into a head.
Flower: Usually bisexual. Zygomorphic, pentermerous and perigynous. Usually Bracteate, Bracteolate.
Calyx: Sepals 5, gamosepalous, usually unequal and the aestivation is valvate. The odd sepal is always anterior in position.
Corolla: Petals 5, polypetalous, irregular and papilionaceous of the 5 petals, posterios petal is larger called the standard petal below the standard, there are two wing petals and two boat shaped keel petals. Usually the keel petals enclose the essential organs. Descendingly imbricate in aestivation.
Androecium: Usually 10 stamens. Monadelphous as in Crotalaria or diadelphous when diadelphous, 9 stamens areunited and one is tree as in Tephorosia, Clitoria etc. In the case of Aeschynomene the stamens are arranged in two bundles and each bundle has 5 stamens. (so 5+5) Dimdorphism is reported in Crotalaria verrucosa where 5 stamens are longer and 5 are shorter. Anther dithecous.
Gynoecium: Monocarpellry, and the ovary is half superior, Unilocular with one or two vertical rows of ovules on marginal placentation. Style long and the stigma is simple and some times feathery.
Fruit: Fruit is a legume dehiscing alongtwo sultures. Some times a samara as in piterocarpus. Seeds are usually nonendospermous. Raphe is usually present.
Pollination Mechanism in Papilionaceae
1. The stamens and stigma emerage out of the keel petals due to an insect sitting on the wing petals and they again return within when the insect flies off. Eg: Trifolium, Crotalaria.
2. Here the essential organs are combined under tension and due to an insect sitting on the wing petals, explode out of the keel petals. Eg: Ulex, Media ago.
3. In certain flowers there is a piston mechanism which squeeze the pollen out of the keel in small quantities.
4. There is a brush mechanism in certain flowers. A brush of hairs upon the style sweeps the pollen in small portions out of the apex of the keel. Eg; Pisdum, Lathyrus.
Popular Examples and economic Importance
In the following members seeds are useful:
1. Arachis hypogea (Groundnut-Tam: Verkadalai, ground nut oil is extracted by crushing the cotyledons. The cake that is left behind during crushing forms a good food for the cattle. The ground nut oils used for cooking.
2. Cajamus indicus (Dhal – Tam: thuvari)
3. Cicerarietinum (Bengal gram – Tam: kndaikadalai)
4. Dolichos biflorus (Horse gram- Tam: Kollu)
5. Pisum sativum (Pea – Tam: Pattani)
6. Phaseolus radiatus (green gram- Tam: Pasip payaru)
7. Phaseolus mungo (Black gram – Tam: Ulundu)
8. Phaseolus lunatus (Butter beans)
9. Trigonella foemum – graceum ( Fenugreek – Tam : Venthayam)
In the following members, the leguminous fruits are used as vegetable.
1. Canavala ensiformis (Sword bean – Tam: Thampattavarai) 2. Cyamopsis tetragonaloba (cluster bean- Tam: Vaalavarai) 3. Dolichos lablab (Country bean -Tam : Avarai) 4. Vicia faba (Beans)
Timber yielding plants. 1. Pterocapus mursupium: (Tam: Vengai maram) Wood useful for making furniture and agriculture implements.
2. P.Santalinus wood is useful for making pillars and carvings. 3. Dalbergia latifolia (Rose wood), D.sissoides: wood useful for making furniture. 4. Aesch nomene aspera (pith plant-Netti) Hydrophyte. Wood is very light and the aerenchymatous stem is useful for making models, fishfloats, and also for section cutting in the laboratory. 5. Butea frondosa “Flame of forest” Tam: Chendurappu.) Light wood is useful. It forms a host for the lac insect which secretes shellac. It is used for making buttons, electrical insulators phonograph records etc.
1. Abrus precatorious: (Tam: kundumani) Red seeds with black spot used as jewller’s weight.
2. Clitoria ternatea: (Tam: Sangupushpam) stem twiner with ornamental white or blue flowers.
3. Crotalaria juncea: (sun-hemp) The best fibres are useful in the coir industry.
4. Erythrina indica: Quickly growing tree (Tam: mulmurungai) cultivated as hedge plant.
5. Glycyrrhiza glabra: Populary known as Athimathuram Rots are medicinal.
6. Indigofera tinctoria; Indian dye is extracted from this plant.
7. Pongamia indica; (p. glabra) (Tam: pungamaram) wood is useful for making cart wheels.
1.7 RUBIACEAE 450/5500 SPS.
Series: Inferae Cohart : Rubiales
Habit: Trees – Cadamba, Morinda tintoria (Tam: Manjanathi) Shrubs – Mussaenda, Ixora (Tam: Iddly poo) Herbs – rubia, Gallium, Oldenlandia. Twiner – Manettia climber – Uncaria (Hooked peduncles) Epiphyte – Hymenopogon parasitious.
Leaves: Simple opposie decussate or Whorled (Gallium) stipulate. The stipules of this family shows a variety of form.
Fig.1.7 Ixora Coccinea
a) Intrapetiolar as in Gardenia (b) nter petiolar as in Ixora, Morinda (c) Leafy stipule as in Rubia, Gallium (d) Hairy stipule as in Dentas (e) Sheath like as in Gardenia sp.
Inflorescence: Mostly cymose type of Infloresence. It may be either Monochasial or Dischasial cymose panicles as in Cinchona, Solitary as in Gardenia.
Flowers: Regular Bisexual, Tetramerous or pentamerous flowers. Epigynous. Rarely imperfect sterile flowers as in Gardenia turgida. Zygomorphic as in Henriquezia.
Calyx : K (4) as in Gallium, Asperula, Lxora, Oldenlandia K (5) as in Coffea, Rubta, Gamosepalous calyx tube adnate to the ovary. In Mussaenda one of the five sepal is muchlarger, leaf like and is brightly coloured in order to attract the insects. Persistent calyz as in Hamelia and Cardenia.
Corolla: C(4) Gallium, Ixora, Oldenlandia etc. C(5) Rubia, Coffea, etc. Gamopetalous, Valvate, twisted or imbricate in aestivation C 5-11 in Gardenia.
Androecium : A 4 or 5 Equal to the number of petals and situated at the throat of the corolla tube ordeep down. Stamens are free and epipetalous. Filaments are short or more or less absent as in Ixora and Coffea.
Gynoecium: G(2) Bicarpellary syncarpous, bilocular G-Gardenia rarely one to may celled – ovules one to many on axile placentation. Style is simple and stigma is generally two lobed. Ovary in inferior. Usually fleshy disc is present
Fruit:Berry – Ixora. Drupe – Gardenia, Coffea. Capsule- Uncaria,
Pollination: Usually pollinatded by Bees, Butterfly and Lepidopteron insects Heterostyly is common. Nectar is usually secreted by a distinct annular nectariferous disc present around the base of the style.
Gohart: Lamiales Series : Bicarpellatae
Labiatae comprise about 170 genera and 3,000 species.
Habit: Mostly mesophytes. Annual or perennial herbs (Leucas, Coleus, etc). A few are more less shrubs. Plants are aromatic, because volatile oils are secreted by glands in stem and leaves.
Stem: Stem is usually quadrangular (Square), hairy.
Leaves: The leaves are simple, exstipulate, opposite, decussate, or less commonly whorled. The margin is entire or toothed. Venation is pinnately reticulate.
Fig. 1.8a Leucas aspera
Inflorescence, It is usually a verticillaster which is present in the axils of opposite leaves (Leucas) In Salvia, three-flowered cymes occur in the axils of opposite leaves. In Ocimum, it is a raceme of cymes i.3., thyrsus. The inflorescence is a panicle n Hyptis
Flower: Bracteate, 1-2 bracteoles may be present or absent. The flowers are complete, bisexual pentamerous, hypogynous and and z(geomorphic. Rarely regular and tetramerous as in menthe sp.
Calyx: 5 Sepals. Usually gamosepalous, tubular, bell-shapped or at times bilabiate having 5 or 10 longitudinal ridges. In Leucas calyx tube ends in 10 rigid teeth. The aestivartion is valgvate or imbricate.
Corolla: The corolla is typically gamopetalous, bilabiate, irregular in various ways. The usual arrangement is 2/3 with posterior pair of petals forming the upper lip which is flat or concave (Salvia) or hook like( Leucas) and the lower lip formed by three petals, of which the middle one is larger. In ocimum, Coleus the corolla shows 4/1 arrangement. The lobes of the corolla are imbricate.
Androecium: It consists of 4 stamens. They are epipetalous, didynamous with the anterior pair of stamens longer than the posterior pair (Leucas). Rarely, the posterior pair of stamens are longer. In Salvia, only the anterior pair of stamens are fertile and the posterior pair are reduced to steminodes or absent. The anthers are dithecous and the connective may be well developed separating the two antherlobes as in Savia. Anthers are divaricate in several cases. Dehiscenceis longitudinal and introse.
Gynoecium: Bicarpellary and syncarpous pistil. Ovary superior, nectar-secreting disc, is present. During the early stages of development, a constriction appears in each carpel dividing into tow loculi and so ovary becomes deeply 4-lobed. Style arises from the base of the ovary between sthe lobes and it is known as gynobasic style. Ovary is tetralocular. Stigma is bifid.
Fruit; A group of 4 achenes. The seeds are non-endospermous.
Economic – Importance:
1. Ocimum Sanctum – (Tam : Tulasi) has medicinal value.
2. Leucas aspera – (Tam. Thumbai0 has medicinal value
3. Mentha piperita – Yields peppermint essence.
4. Salvia sp. – Garden plants.
5. Mentha atriviridis – (Tam. Puthina keerai).
1.9 PALMAE (ARECACEAE)
Palmae consists about 200 genera and 1500 species which are mostly tropical and subtropical in distribution.
Habit: Mostly trees with a tall, woody stem which is unbranched, bearing a crown of leaves at the top in a radial manner. The stem is covered by persisting leaf bases or marked by scars left by leaf bases.
Leaves: The leaves are large, having a broad petiole which ensheaths the stem. The old leaves may fall down but leaf bases may be persistent on the stem. The leaves are feather bases may be persistent on the stem. The leaves are feather like (pinnate) a in Cocos or fan like) palmate) as in Borassus.
Fig 1.9a Phonex sylvestris
Inflorescence: It is usually a spadix. The inflorescence may be unbranched (Borassus) or branched spike(Cocs) with a large common bract or spathe.
Flowers: They are usually regular, sessile, trimerous hypogynous, unisexual, monoecious (Areca, Cocos etc.,) or dioecious (Borassus). They are rarely bisexual. If they are monoecious, female flowers are at the base. In some cases the male and female flowers may be mixed as in Geonoma.
Perianth: They are 6 in number leathery parts which may be valvate or imbricate. They are sepaloid and are arranged in two whorls. The outer whorl may be smaller in size (male flowers of coconut) or may be copular as in Calamus.
Androecium : There are 6 stamens arranged in two whorls, alternating with the perianth members. Rarely, they may be numerous as in Caryota, etc. the stamens have linear dithecous anthers and dehiscence is longitudinal.
Gynoecium:Tricarpellary, syncarpous pistil with trilocular superior ovary with single basal ovule in each locule. The placentation is axile (Cocos). In Cocos of the three carples, only one carpel is fertile and the other two carpels are sterile. In Areca, the ovary is on3-celled with a single erect ovule. There are three free carpels in phoenix with one ovule in each carpel. A pistillode may be present in male flowers (Cocos) and staminodes may be present in the female flowers (Calamus)
Fruit: It is a large drupe in coconut. In Borassus, it is a drupe. It is a small berry in some cases.
Seeds: The seed is oblong in date palm. The endosperm may contain oil as in Cocos. In Areca, the endosperm is ruminate.
The Kenel of Cocos nucifera (coconut) is edible and coconut oil is extracted from the dry Kernel. Fruits of phoenix sylvestris (date) and P. dactylifera are edible. Endosperm of Areca carechu is used as betel nuts. The fleshy endosperm of Borassus flabellifer is edible. 2. The sugary sap obtained by cutting the young inflorescence of B rassus flabellfer is fermented to yield toddy or arrack. 8. Palm jaggery is manufactured from the sugary sap of date (phoenix sylvestris) and Toddy palm (Borassus flabellifer) 4. Sago is prepared from the starchobtained from the pith of Metroxylon sago 5. Fibre obtained from the mesocarp of Cocos nucifera are used to manufacture coir and it is used in making ropes, mats and in suffing. 6. The stems of Cocos nucifera and Borassus fliabellifer are used for construction work.
1. Cocos nucifera (coconut) – (Tam: Thennai maram)
2. Borassus flabellifer (Tam: panai maram)
3. Areca catechu (Tam : Paakku)
4. Phoenix dactylifera (Tam : Peichcham)
UNIT – II
2.1. ULTRA STRUCTURE OF A PLANT CELL
Fig. 2.1 Ultra Structure of a Plant Cell
Mitochondria are the most important organelles of the cell. The mitochondrial system is responsible for system is responsible for generating energy in the form of ATP. Therefore the mitochondrion is called as “Power house” of the cell. The mitochondria occur almost in all living cells (plant and animal cells). But they are absent in mature red blood cells of multicellular organisms. In an average cell there are several hundreds of mitochondria. The size of an average mitochorndrion is about 15,000 Angstrom units. (An Angstrom unit is a ten- millionth of all millimeter.)
Fig 2.2 Structure of Mitochonidria
The mitochondrion is bounded by a two layered membranous envelope, trapped like a thermosflask with a watery fluid fillingup the space between them. This fluid provides a communication between the two layers and also supplies the enzyme in the membranes. The outer membrane is stretched tightly over the mitrochondrion and the inner membrane by repeated infoldings forms numerous finger-like projections called as cristae. The inside of the mitochondrion is filled up with a fluid matrix F.S. Sjostrand and Fernandez Moran discovered the presence of thousands of particles or granules attached to the membranes. The particles are rougly 30 Angstrom units in size. They are the elementary units that carry out the chemical activities of the mitochondrium.
Functions of mitichondria: They act as the chemical factories in which cellular respiration is carried out. During this process the energy locked up in food material like carbohydrates proteins etc, are released and stored up in the form of ATP. The ATP is called as the ‘energy currency’ of a cell and is utilized whenever it is required in the organism.
The plastids are protoplasmic bodies of all plants except blue – green algae and bacteria. They play an important role in plant metabolism.
Types of plastids: They are classified on the basis of the presence or absence of colour of their pigments. The colourless ones are called leucoplasts, the green coloured ones are called chlropolasts, and the coloured plastids coloured other than green are called as chromoplasts.
Fig 2.3 Structure of the chloroplast
a. The Chorloplasts: They are called as green plastids because they contain green coloured colouring pigments. They are very important because they are the centres of photosynthesis.
The Chloroplasts vary in their shapes. They are cup shaped in chlamydomonas. Plate like-or-girdle shaped in ulothrix, ribbon shaped in spirogyra, star shaped in zygnema, reticulate inoedogonium. But in higher forms they are more or less uniform in shape; commonly spherical or ellipsoidal and small in size. They are about 5-8 microns in diameter.
Ultra structure of chloroplast: The chloroplasts revealed the detailed structure of chloroplast. The chloroplast is bounded on its outside by a smooth membrane (outer membrane) that shows osmotic properties. In a mature schloroplst inside the outer membrane, grana is present embedded in a uniform granular matrix the stroma. About 50 grana may be present in each chloroplast. (See chapter photosynthesis for diagram).
A single granum is compose of variable number of membrane elements stretched almost parallel, one over the other, to form an orderly and usually cylindrical array that resembles a pile of coins, in which the pigment chlorophyll is allocated. The grana are connected to one another by simple inter granal membrane, that run throughout the choloroplast and form the stroma.
The single membrane element in the granum is called as “Unit membrane” by Robertson. The unit membrane is triple layered (protein-lipid-protein). There is actually two unit membranes, which envelope the chloroplasts (double membrane). Such a pair of unit membranes are called as a ‘lamellar unit’. Mary number of these lamellar units are pilled over one another to form the chloroplast granum of higher plants.
The granum, in the chloroplast possesses the biochemical machinery of photosynthesis. The pigment chlorophyll of the chloroplast is located in the triple layered membrane units of the individual lamella. It is present between the lipid and the protein components of the unit membranes.
b. Chromoplasts: Chromoplasts are coloured plastids. Their colour is other than green. These plastids contain carotenes and xanthophylls. They do not possess chlorophyll. They are found in coloured parts of plants such as the flowers and fruits. The yellow colour in the petals Tribulus and the red colour in the capsicum fruit wall and Nerium flowers is due to the presence of chromopasts in them.
c. Leucoplasts: The leucoplasts may transform themselves into chloroplasts or plastids of other colours when exposed to light. Recently it is discovered that the leucoplasts are concerned with the formation of cellouse in higher plants. The cell wall formed during cell division, is formed by cementing together of cellulose particles produced by leucoplasts.
2.4 ENDOPLASMIC RETISCULUM
The cytoplasm is traversed by a network of interconnecting tubules which are known as endoplasmic reticulum or ER. It has a vast inter-connected cavity which is bounded by a single unit membrane. There are two types of ER. 1. Smooth surfaced ER where the outer membranes are smooth 2. Rough surfaced ER, where they have attached ribosome (Fig.2.4).
Fig 2.4 Structure of the Endoplasmic reticulum
Functions of ER: 1. The ER forms an ultra structural skeletal framework of cytoplasm. 2. It provides mechanical support t the cytoplasm, 3. It acts as an intracellular circulatory systems and it circulates various substances into and out of the cells by the membrane flow mechanism. 4. It synthesizes lipids, glycogen, hormones etc.
It is the most important organelle of the cell. The presence of nucleus was first discovered by Robert Brown (1831) Usually every living cell contains one nucleus(uninuleate). But sometimes two (Binucleate) or many (multinucleate) nuclei may be present. The term coenocyte is often used to designate a multinucleats cell. Rarely a cell may be without a nucleus in adult condition (eg. Phloem tubes) (Fig.2.5).
Fig. 2.5 Structure of a nucleus
Size and Shape: The nucleus shows variation in size, shape and position. An average nucleus varies from 5 microns to 25 microns in size. Usually the, nucleus is spherical or ellipsoidal in shape in almost all plants, through other shapes are also met with in some.
Structure: The nucleus is bounded by a membrane. Inside the nucleus the following structures are present: (1) Nuclear sap (2) Chromonemata and (3) Nucleolus.
Nuclear membrane: This membrane separates the nucleus from the surrounding cytoplasm. It withstands pulling and pushing in the cytoplasm of the cell. It is actually a double layer. The endoplasmic reticulum is no continuity with the outer of the two membranes. There are many minute pores on this membrane. Thus the inside of the nucleus is continuous with the cytoplasm outside nucleus with in the cell and thus they govern the vital traffic of material between cytoplasm and the nucleus.
Nuclear sap or Karyolymph: It is an achromatic jelly like fluid.
It is denser than the cytoplasm and it differs from it in that there are no vacuoles in the nuclear sap.
Chromatin reticulum: In a metabolic or living nucleus. Which is also called es ‘resting nucleus a solid material of chromatin is suspended. It is called as Chromatin ‘reticulum’ or network. It is made up of a specific number of slender elongate and crooked threads the chrmonemata. They are the prochromosomes in a dividing cell they become thickly coated with an additional nucleoprotein matrix, become thick and short and become chromosome.
Nucleolus: Each nucleus possesses a definite number of nucleoli (Usually one or many). They are present in a non dividing nucleus and they remain attached to specific chromosomes at fixed points. But they disappear during each cell division and reappear in metabolic nucleus.
The nucleolus is a spherical body. Under E.m. it appears as a collection of granules that are approximately of the size and sappearance of ribosomes. Infact they are believed to be ribosomes which are synthesized in the nucleus and are extruded through the nuclear pores into the cytoplasmic matric. Outside the nucleus they are presumed to remain attached or unattached to the endoplasmic reticulum (ER).
2.6 MONOHYBRID CROSS
When crosses involving only one pair of factor are called monohybrid crosses. In case of a monohybrid cross the phenotype ratio of 3:1 obtained in F2 generation is called monohybrid ratio. Mendel’s law of segregation falls under monohybrid crosses.
LAW OF SEGREGATION
The fact that some dwarf plants appeared in the F2 generation clearly indicates that the factor for dwarfness was transmitted through the tall F1 plants unaffected by its close association with the factory for tallness. It simply skipped a generation.
When the F1 hybrid forms gametes (or spores in plants), the factors of an allelomorphic pair separate of segregate from each other.
For example, a gamete may carry the factor for tallness or for dwarfness but it cannot carry both; a factor for coloured flower or for white flower, but never both. This important deduction is known as the law of segregation of characters or Mendel’s first law of heredity.
According to this law, the F1 hybrid tall peas of Mendel’s experiment will produce two kinds of gametes, some carrying the factor for tallness and an equal number carrying the factor for dwarfness. As the gametes will pure for tallness or for dwarfness, the law is also known as the law of purity of gametes purity, that is, with reference to one member of an allelomorphic pair. Let us see how Mendel’s law explains the observed numerical results of his experiments.
It will be evident that the original tall parent will have two genes for tallness TT, having received one from each of its two parents. Similarly, the dwarf parent will have two genes tt. The gametes formed by the tall parent will each have a gene T and the gametes of the dwarf parent will have a gene teach. Their union will form an F1hybrid carrying the factors Tt. This hybrid will be tall because T dominates over t. When the F2 hybrid forms gametes, the two factors T and t separate from each other and the result is the formation of two kinds of gametes in equal numbers, half carrying the factor (gene) T and half carrying the factor (gene) t. This is in accordance with Mendel’s law of segregation of characters. As a result, two kinds of pollen- grains and two kinds of ovules are formed in the hybrid pea. The pollen-grains T has an equal chance of fertilizing an ovule T or t, producing seeds with TT or Tt combination. When the number of fertilizations is very large, as happens in nature, the kinds of seeds produces by the T type of pollen- grains will be approximately equal. Similarly, the pollen- grains of the t type have an equal chance of fertilizing the ovules T or t, producing Tt and tt seeds in equal number. The results to be expected on theoretical considerations are shown in figure 2.4. The F2 generation will, therefore, consist of three kinds of plants in the following ratio:
1. Pure tall (TT): 2 hybrid talls (Tt): 1 dwarf (tt) Or 3 talls: 1 dwarf.
On the basis of this law, Mendel had predicted that of the F¬2 tall plants one-third will produce only talls and two-thirds will produce a mixed progeny of talls and dwarfs in the ratio of 3: 1 in the F3 generation. These results obtained from theoretical considerations on the assumption of segregation of characters are in complete accord with the numerical data obtained in actual experiments. No other explanation of the behaviour of hereditary traits through successive generations of hybrids has yet been given.
Mendel’s laws have been found to hold good in the case of animals also. Thus, when a pure black guinea pig (a small nearly tailless South American pet rodent) is mated with a white guinea pig, the animals produced in the F1 generation are all black. They are hybrids. When any two hybrid black guinea pigs are mated, the animals produced in the F2 generation will be three blacks. 1 while. The blacks are of two kinds: one- third are pure black that breed true and two-thirds are again hybrids which when inbred produced a mixed progeny (F3) of blacks and whites in the ratio of 3:1. The results are shown in Figure 2.5.
P TT tt
Generation Tall Dwarf
Gametes of T x t
1 TT = pure tall
2 Tt = hybrid tall
t 1 tt = dwarf
Mendel’s law of segregation may be defined as the non-blending of alleles in the hybrids. This is the most fundamental principle in all heredity what is means is this that if a good gene and its bad allele come together in a hybrid they will not permanently influence each other even if they continue to be together in hybrids, generation after generation.
2.7 DIHYBRID RATIO:
The experiments described above were carried out with reference to a single pair of contrasting characters. Mendel continued his experiment with reference to two pairs of characters, the colour of the cotyledons and the structure of the seed coats. Some of the peas had yellow-coloured cotyledons and others green-coloured cotyledons ; some were round and others were wrinkled. Character for character yellow was dominant over green and round over wrinkled, so that a plant breeding true for yellow-coloured cotyledons and round coats of seeds, may be have the hereditary constitution, YYRR, and a plant breeding true for green-coluored cotyledons and wrinkled coated seeds, may be said to have the hereditary constitution, yyrr. Two such parents were chosen and crossed. In the hybride of the first generation, all the plants produced seeds that had yellow cotyledons and round coats. When plants produced seeds that had yellow cotyledons and round coats. When plants grown from these were allowed to self-fertilize, there were produced not only types that were similar to the parents but also new types : there were produced plants that bore yellow-octyledoned and round-coated seeds, yellow-cotyledoned and wrinkled-coated seeds, green-cotyledoned and round-coated seeds and green-cotyledoned and wrinkled-coated seeds. Of these, 315 had yellow-cotyledoned and round-coated seeds, 101 green-cotyledoned and wrinkled-coated seeds, 108 green-cotyledoned and round-coated seeds and 32 green-cotyledoned and wrinkled-coated seeds. This works out as
9 : 3 : 3 : 1.
Male gametes RY Ry rY RY
Female gametes RY RY RY
Round yellow RY Ry
Round yellow RY rY
Round yellow RY ry
Ry Ry RY
Round yellow Ry Ry
Round green Ry rY
Round yellow Ry ry
rY rY RY
Round yellow rY Ry
Round yellow rY rY
Wrinkled yellow rY ry
Ry ry RY
Round yellow ry Ry
Round green ry Ry
Wrinkled yellow ry ry
Phenotypes : 9 round yellow ; 3 round green ; 3 wrinkled yellow ; 1 wrinkled green.
Here, therefore, are new combinations of characters, which are not met with in the original parental types. Mendel was quick to see that the 9 : 3 : 3 : 1 ratio which he got during his study of the inheritance of two pairs of characters was only a modification 3 : 1 ratio that he noticed in his study of the inheritance of a single pair of characters. He counted seeds of the same coloured cotyledons leaving out the nature (round or wrinkled) of the seed coat and found that there were nearly thrice as many yellow (416) as there were greens (140). Similarly, when he coloured and wrinkled seeds, leaving out the colour of the cotyledons, he obtained the same ratio : 423 round and 133 wrinkled. Algebraically, 9 + 3 + 3 + 1 is (3 + 1)2.
From the results, he enunciated his famous laws of heredity : (1) the law of segregation of characters and (2) the law of independent assortment of characters. According to the first law, the ‘determiners’ or factors for each character maintain their identity in the body and at the time of gamete (or spore) formation separate, so that the gamete comes to have a single factor only. We may therefore speak of a gamete as being pure for a character, while a zygote may or may not be. The concept of the ‘purity of gametes’ is an important contribution of Mendel.
According to the second law, which follows as a corollary of the first, the characters (i.e., the determiners or factors) of an individual are handed down to the gamete independent of one another, since each gamete can carry only one factor of a pair. The random mating of gametes gives rise to zygotes possessing combinations of characters that were not originally met with. This is amply borne out by the appearance of yellow wrinkled, and green round seeds. In fact, this concept of recombination of characters, is one of the foundations upon which the entire edifice of genetic is built.
UNIT – III
3.1 SIMPLE PERMANENT TISSUES
Simple permanent tissues the constituent cells are homotgenenous (similar) in structure and function. They are uniform in every respect. Parenchyma, collenchyma and sclerenchyma are examples of simple tissues.
(a) Parenchyma: The simple kind of permanent tissue is the parenchyma. It forms the bulk of the softer parts of the plant body. It consists of polygonal or rounded cells which are equally expanded on all sides. The cell wall is thin and is made up of cellulose. At the corners where three or four cells meet, there are gaps called intercellular spaces, filled with air (Fig.3.1a). The cells of the parenchyma are all living and the cytoplasm is often reduced to a layer lining the cell wall. This tissue serves for storage, conduction and also for the manufacture of food materials. In leaves, the parenchyma cells contain numbers chloroplasts and are known as chlorenchyma cells. In the floating roots of Jussiaea, the parenchyma cells are widely separated from one another by very large air spaces. Such a loose parenchyma tissue is celled aerenchyma.
Fig. 3.1a Parencyma Fig 3.1b Collenchyma
(b) Collenchyma: In herbaceous stems, rigidity is given by a special tissue called collenchyma. The cells composing this tissue are slightly elongated. Their walls are very much thickened at the corners due to heavy deposit of cellulose so that the cells look polygonal in cross-section. (Fig.249). The cells are living and may contain chloroplasts, when they are also assimilatory in function. Collenchyma may occur either as a continuous band or as isolated patches.
(c) Selerenchyma: This tissue strengthens the older parts of the plant. The cells composing it are considerably elongated and have tapering ends. The cell walls are very much thickened and the cell cavity is very small. They are all dead cells, all the living substance heaving been used up in thickening the wall. The wall is also lignified. Because of their elongated narrow from, the cells are also known as sclerenchyma fibers. (Fig 3.1c)
Fig 3.1c Selerenchyma Fig 3.1d Stone cells
Stone cells are sclerenchyma cells which are not elongated like the fibres. They are more or leas equally development on all sides and have very thick, lignified and stratified walls. There are, however, some unthickened areas in the walls, celled pits. These pits are very much branched and appear like a system of canals opening into the narrow cell cavity. (Fig. 3.1d) Stone cells may be found either singly or in groups in any parts of the plant. The hard, gritty portions of fruits like the pear consists of groups of these cells.
Collenchyman and sclerenchyma are also known as mechanical tissues since they give support and rigidity to the plant body.
In complex tissues the constituent cells are not homogeneous in structure and function. Xylem and phloem are examples of complex tissues.
As the xylem is composed of different types of cells, the xylem is a complex tissue. It is made up of xylem vessels, xylem parenchyma and fibres. Each vessel is long continuous tube formed by the fusion of a vertical row of elongated cells and the dissolution of the cross walls separating them. The contents of the cells are spent up in the thickening lignification of the wall. The xylem vessels therefore composed of dead cells. The vessels carry and mineral salts from the roots to the leaves.
Fig. 3.2 a Xylem vessles Fig 3.2b Bordered Pits
In order keep the vessels open and prevent their collapse the walls of the vessels are thickened the inner side in various ways. The vessels which are formed first are strengthened by the formation of a number of ring-like thickenings. These vessels are known as annular vessels. The vessels formed a little later show thickenings in the form of spiral bends and are called spiral vessels. Bigger vessels with the thickening forming a network are produced still later, and these go by the name reticulate vessels (Fig 3.2a). The last formed vessels have their walls uniformly thickened except for small, rounded, thin areas which look like so many pits. Because of this these vessels are known as pitted vessels (fig 3.2b). The pits in this case are known as simple pits. A different kind of pits is the bordered pit. While in the simple pit, the cavity is more or less cylindrical, in the bordered pit, due to the overgrowth of the thickening material above the cavity, so as to form a project border, the cavity is somewhat funnel-shaped. As with the simple pits, here too the pits are formed in pairs, one on either side of the cell wall. In the simple pits, the cell wall between the pair remains then. But in the bordered pits, the wells shows a thickening in the middle. This thickening helps in regulating the flow of water and mineral substances thought the pits.
The xylem vessels are known as the tracheae. Different kinds of conducting elements known as the tracheids are met with in the xylem of ferns and gymnosperms. Unlike the vessels which are long continuous tubes, these tracheids are elongated cells with short tapering ends. They too are dead cells with strong lignified walls exhibiting the various kinds of thickening like the vessels. Bordered pits are very common in the tracheids.
Xylem parenchyma is found surrounding the vessels. The cells are like parenchyma cells but have thicker walls and appear rectangular in cross-section. They serve for storage and also furnish mechanical strength.
The wood fibres are narrow, elongated, thick-walled, dead cells with pointed ends. They occur with the xylem parenchyma and give mechanical strength.
This too is a complex tissues composed of sieve tubes, companion cells and phloem parenchyma. The sieve tubes are long tubular structures whose mode of formation is similar to that of the vessels. But the cross walls separating the cells instead of breaking down, develop numerous small openings so as to look like a sieve. Hence each perforated cross wall is called a sieve plates. The cells composing the sieve tubes retain their protoplasmic contents but have no nuclei. Through the holes in the sieve plates the cytoplasm of one cell communicates with that f the next. The contents of the sieve tubes are rich in food materials and the tubes serve for the conduction of these to the various parts of the plant. Unlike the vessels, the tubes are thin-walled.
Fig 3.3a Sieve tube and companion cell
In ferns and gymnosperms the component sieve elements retain their individuality. Their sieve areas, i.e., areas of perforation are not clearly differentiated from one another and therefore definite sieve plates cannot be made out. These, in contradistinction to sieve tubes, are termed sieve cells and they correspond to the tracheids of xylem. The sieve cells are long and slender, tapering at their ends and have steeply inclined walls.
Associated with the sieve tubes are found the companion cells, which are elongate, thin-walled and narrow. Each cell has a prominent nucleus and dense cytoplasm. The companion cells are supposed to control the activity of the sieve tubes. (Fig.3.3a) But in ferns and gymnosperms companion cells are absent.
The phloem parenchyma surrounds the sieve tubes and companion cells. The cells composing it are thin-walled as in ordinary parenchyma but are smaller in size and vertically elongated.
3.4 STRUCTURE OF THE DICOT STEM
A transverse section (T.S) of the sunflower stem shows the following arrangement of tissues under the microscope:
Epidermis: It is the outermost layer of the stem. It is made up of a single row uniseriate) of rectangular living cells Their radial walls are united closely to one another. The epidermis has a thick cuticle. The intercellular spaces are absent. Multicellular epidermal hairs develop from a group of epidermal cells. Many stomata are persent on the epidermis. Epidermis forms the protective layer of the stem.
Fig. 3.4a T.S of Dicot stem
Cortex: It lies just below the epidermis. It is made up several layers of tissues. It is differentiated into three regions, collenchyma, parenchyma and endodermis Collenchyma tissue consists of four or five layers of cells and is found below the epidermis. The cell corners are thickened leaving no intercellular spaces. Some of the cells contain green chloroplasts and they are called chlorenchyma. Collenchyma tissue gives mechanical strength to the plant. The parenchyma cells are found in between the collenchyma and the endodermis. This region is formed of many layers of thin walled cells. The cells are oval or spherical in shape with conspicuous intercellular spaces. A few mucilage ducts are also present in this region.
The endodermis is the innermost limiting layer of the cortex from the stele. The cells are characterized by the presence so starch grains and so the endodermis is also called as the starch sheath. The cells of the endodermis are living, barrel shaped and arranged in a compact manner without intercellular spaces.
Pericycle: It is present just below the endodermis. It is composed of several layers of cells. The cells are two kinds:
(1) the cells, outside the vascular bundles and below the endodermis, are thick walled and sclerenchymatous: (2) The cells in between the vascular bundle are parenchymatous. Storage and protection are their functions. The bundles are arranged in a ring. Each bundle has three important parts. They are the xylem, the phloem and the cambium. The xylem and the phloem lie together on the same radius and hence the arrangement is called as collateral; the cambium is present in the vascular bundle, and so it is said to be open. The phloem lies between the sclerenchyma and the cambium. It consists of sieve-tubes, companion cells and phloem parenchyma The phloem is responsible for translocation of food materials from the leaves to the storage organs also to the different growing regions. The cambium is present between the xylem and the phloem. It consists of thin-walled, rectanglar cells without intercellular spaces the cambial cells are living and meristema’ic in nature. They are responsible for secondary growth of stem in thickness. The xylem lies internal to the cambium and consists of the following elements in the sunflower plant: vessels, xylem parenchyma and xylem fibres.
The first formed xylem vessels sare smaller andthey constitute the protoxylem which lies towards the centre of the pith region, while the bigger vessels, formed leter, constitute the metaxyiem found near the cambium This arrangement is called as endarch The xylem vessels conduct water and mineral salts to the leaves and give mechanical strength to the plant.
Medullary rays: The medullary rays are composed a few layers of radially elongated cells, lying in between the adjacent vascular bundles. Their main function is the conduct water and food materials out-ward or in a radial direct on.
Pith: It is present in the centre of the stem and is surrounded by the vascular bundles. The cells ae thin-walled and parenchymatous with large intercellular spaces. The function of pith is to store the food and to conduct water.
3.5 ANATOMY OF DICOT ROOT
A transverse section of a young sunflower root shows the following arrangement of tissues from the periphery to the centre.
Epidermis: It is also called as piliferous layer. It is uniseriate (single layered) consisting of living cells. The cuticle is absent some of the epidermal cells grow out as unicellular root hairs. The root hairs are concerned with absorption of water and mineral salts from the soil.
Cortex: It is present below the piliferous layer and consists of several layers of thin-walled parenchymatous cells with plenty of intercellular spaces. These cells often contain leucoplasts and starch grains. There is no collenchyma or sclerenchyma in the cortex of the root. Chloroplast is totally absent. The cortex is demarcated from the stele by a single layer of well defined cells, the endodermis. It forms a distinct layer in the roots. The radial walls of the endodermal cells are thickened.
Stele: The stele consists of the pericycle, conjunctive tissue and the vascular bundle.
Fig 3.5 T.S of Dicot Root
Pericyle: It is made up of a single layer of cells and is found below the endodermis. The cells of the pericycle sare generally thin-walled and parenchymatous. In sunflower, the pericycle is made up of sclerenchyma cell and is seen in patches].
Conjunctive tissue: It is made up of thin-walled parenchymatous cells occurring between the xylem and the phloem bundles.
Vascular bundles: The number of xylem and the phloem bundles are limited in roots. They alternate with one another in different radii as separate bundles. Since they are arranged in alternate radii, it is called radial arrangement. The number of either the xylem or phloem bundles is usually four in roots and hence it is termed as tetrarch. The protoxylem is found towards the periphery and the mataxylem towards the centre and so this arrangement is called exarch. (Remember it is endarch in stem) The phloem consists of sieve-tubes, companion cells and phloem parenchyma.
Pith: In the young plant, the centre of the root is occupied by meristematic cells which form a part of conjunctive tissue. The meristematic cells later form mataxylem vessels. So it appears as if the young root has pith in the centre.
3.6 STRUCTURE OF THE DICOT LEAF
A transverse section of the sunflower leaf shows the following arrangement of tissues.
Upper epidermis: It consists of a single layer of cells with a thick cuticle. In this layer, the stomata are usually absent. Several multicellular hairs are presnt on it. All the epidermal cells are compactly arranged without any intercellular spaces in between them.
Mesophyll: It is differentiated into an upper palisade parenchyma and lower spongy parenchyma.
Fig 3.6 T.S of Dicot leaf
Palisade parenchyma: it lies just below the upper epidermis. It consists of two layers of elongated, columnar cells. They are very closely packed at right angles to the epidermis. The cells contain plenty of chloroplasts and therefore they are photosynthetic in function.
Spongy parenchyma: It consists of loosely arranged cells. They are irregular in shape. Large intercellular spaces are present in between them. These cells contain less number of chloroplsts. Since the cells are all loosely arranged they are well suited for the easy diffusion of gases better them than of palisade cells.
Vascular bundles: The vascular bundles are clearly seen in the section passing through the mid-rib of the veins. Each vascular bundle except mid-rib in the leaf is surrounded by a sheath of parenchyma known as the bundle sheath. The bundle is collateral and conjoint. The xylem is found towards the upper epidermis and phloem towards the lower
Lower epidermis: like the upper epidermis, it also contains a single layer of cells. But it differs from the upper epidermis only in having numerous stomata. The main function of both the upper and lower epidermal layers is protefction.
Stomata: Very minute openings called as the stomata are present in the aerial parts of the plants, especially in leaves. Each stoma has a minute pore called as the stomatal aperture which is bounded by a pair of bean-shaped epidermal cells called guard cells. Each guard cell contains cytoplasm, a prominent nucleus and chloroplasts. The stoma leads to a large air cavity called the respiratory chamber; which is surrounded by the subsidiary cells and also is in communication with the intercellular space of the leaf. The walls of the guard cells are elastic. The function of these cells is to increase or decrease the size of the stomatal opening, and thus regulate the exchange of gases between the atmosphere and the internal tissues. The guard cells with the help of the chloroplasts can prepare starch by photosynthesis. The stomata act as the passage for respiration, transpiration and photosynthesis.
Stomata occur on all green parts of plants, especially in the leaves and stems. They are many on the under surface of dorsiventral leaves but they are found distributed uniformly on both the surface of the isobilateral leaves.
3.7 STRUCTURE AND DEVELOPMENT OF ANTHER:
In flowering plants a stamen consists of a filament and an anther. Usually the anthers are dithecous consisting of two anther lobes. The connective which is present between the antherlobs forms a column of sterile tissue. Each antherlobe has two enlongated microsporangia separatedby a piece of sterile tissue. Externally, a longitudinal groove is seen between the antherlobes. At maturity, the two smicrosporangia of an antherlobe become merged together due to the breakdown of the partition between them. In some plants each antherlobe has only one microsporangium, e.g..wolffia.
In Malvaceae, the anthers are monothecous. They have only two sporangiaand at maturity, the sterile tissue between the two loculi breaks down so that a single loculus is formed.
Fig 3.7 Microsporogenesis
During the early stage an anther consists of a homogeneous mass of meristematic cells surrounded by an epidermis. During the course development, it becomes fourlobed. The development of the microsporangium is initiated in each lobe with the differentiation of archesporial cells in the hypodermal region. The archesporial cells become distinct because of their age size, radial elongationand conspicuous nuclei. The extent of archesporial tissue varies in length as well as breadth. Normally the archesporium consists of several vertical rows of cells and they appear like a plate of cells in cross section. In some plants the archesporium is formed of a single vertical row of cells is seen in a cross section of the anther as in Boerhaavia. The archesporial cells divide periclinally forming inner row of primary sporogenous layer and an outer row of primary parietal layer. The primary parietal layer forms the part of the anther wall inner to the epidermis. The primary sporogenous cells either function directly or after mitosis, as microspore, mother, cells.
The cells of primary parietal layer undergo pariclinal and anticlinal divisions forming divisions forming 3-4 concentric layers of cells inner to the epidermis. The outermost layer of these which is immediately below the epidermis develops into the endothecum. The cells of endothecium are radially elongated and before dehiscence, they show fibrous bands of thickenings on their walls. The endothecium is very important at the time of dehiscence. Sometimes, endothecium does not show any thickening of fibrous bands as in cleistogamous flowers and in anthers opening by pores. The innermost wall layer gets differentiated into a nutritive tissue called as tapetum. The layers between the endothecium and tapetum are called as middle layers. The cells of the middle layers are stretched rapdly and often found collapsed due to compression.
Tapetum: It is innermost wall layer of the anther and it completely surrounds the sporogenous tissue. Tapetum is of considerable physiological significance because the food material entering into the sporogenous issues must pass through it. The cells of tapetum have dense cytoplasms and one or more prominent nuclei.
Two Types of Tapetum:
1. Secretory or Glandular type: In this type, the tapetal cells remain intact in their original position throughout the development of micropores. The cells secrete substances from the inner faces and finally they break down. This is more common in angiosperms than the amoebied type e.g., Mirablis.
2. Amoeboid or Plasmodial type: In this type, the cell walls break down at a very early stage and the tapetal protoplasts move into theanther cavity. These protoplasts coalese and sform an amoedoid mass or tapetal periplasmodium, which surrounds the spore mother cells. It supplies nutrition to the developing spore mother cells and may sometimes help in the formation of exine. Amoeboid tapetum is found in Typha, Butomus etc.
3.8 STRUCTURE OF THE OVULE
Each ovule is provided with two coats, an outer and an inner. These coats are known as the integuments. In some ovules, however, there is only one integument. Enclosed within these integument, and forming the bulk of the ovule, there is a special kind of tissue called the nucellus. The nucellus is very rich in reserve food materials. Lying embedded in the nucellus, is the most important part of the ovule. This is a sac-like structure (or cell) known as the embryo sac. The ovule is attached to the placenta by a small stalk called the funicle. The two integuments do not cover the nucellus completely, but leave a minute opening at the top. This is the micropyle and it leads to the nucellus. The integuments and the nucellus are free from each other at all places except at the base where they are completely fused. This place of fusion is known as the chalaza. The place where the funicle enters the ovule is called the hilum (Fig.3.8 a).
Fig. 3.8a L.S of Ovule
The embryo sac
The embryo sac is situated in the nucellus, a little below the micropyle. In the middle of the embryo sac, there is a small, highly specialized, spherical body known as the nucleus. This nucleus divides into two. The two daughter nuclei move away from each other and reach the opposite ends of the embryo sac. Then, each of them divides ywice in succession, so that at each end of the embryo sac there are now four nuclei. One nucleus form each end travels to the middle of the embryo sac. The two nuclei meet and fuse together. The fused nucleus is known as the secondary nucleus. The three nuclei at the micropylar end surround themselves with walls so as to form three small compartments or cells. These constitute the egg apparatus. The middle one is the egg (female gamete). The two side ones are known as the helpers or synergids. Similarly, the three nuclei at the other end of the embryo sac also get organized into three cells called the antipodal cells. This is the structure of the embryo sac at the time of fertilization.
Development of the ovule
The ovules arise upon specialized tissue called placenta inside the ovary. The ovule has a nucellus Various forms of ovules are found, the anatropous, the orthotropous, the campylotropous and the amphitropous, sometimes one form intergrading into another.
Ordinarily, the ovule has one or two integuments. The number o the integuments is constant in most families and it is only rarely that we find both unitegmic and bitegmic ovule in one and the same family. A single massive integument is almost universal in the sympetalae. In Archichlamydeae (Polypetalae and Incompleatae of B & H) and monocots, most genera develop two integuments but a few have only one. The unitegmic condition may have arisen either by a fusion of the two integuments or by elimination of one of the two integuments. In some plants, a third integuments, forming what is known as aril is present. Eat another motivication of the integuments is the development of caruncle, caracteristicv of some Euphorbiaceae; it is develop by a proliferation of the integumentary cells at the micropylar region. In Opuntia, the finical which is extremely long surrounds the Ovule and appears like a third integument. The integuments are made up of several rows of cells and normally many of their layers do not take part in the formation of seed coats.
Nusellus is a mass of undifferentiated cells enveloped by the integuments. Soom a cell generally located directly below the epidermis becomes Conspicuous by its larger size, dence cytoplasam and prominate nucleus. This is called primary archesporial cell. The primary archesporial cell appears as if it is a terminal member of a series of nuclear cell because below it the cells or arranged in a row.
The archesporial cell devides to form a primary parietal cell and a primary sporogenous cell. The primary parietal cell may devide and form a variable number of wall layers, or it may remained undivided. The primary sporogenous cell may function directly as megaspore mother cell without undergoing any further divisions.
The megaspore mother cell divides meiotically forming a linear tetrad of megaspores. Of the four megaspore, that’s formed the one at the chalazal end is generally the functioning megaspore, i.e., it gives rise to the embryo-sac. The other three disintegrate.
Development of the Embryo-sac or the female gametophyte
The polygonum type of development of the embryo-sac is what accurse in atleast 70% of the angiosperms. (Fig.3.8b). The functioning megaspore enlarges insize. This enlargements insize is accompanied by increased vaculisation, one large vacuole appearing on either side of the nucleus in the direction of the long axis of the cell. Now, the haploid nucleus under goes a mitotic division and the two daughter nuclei move apart to the opposite poles. At the stage, most of the cytoplasm is confined to points around these nuclei and the rest forms a thing peripheral layer, a large vacuole occupying the center. Each of the nuclei divides twice, resulting in a four nucleate stage at first and a eight nucleate stage latter. In the eight nucleate stage, there is a quartet at the chalazal end and another at the micropylar end. One of the nuclei from each quartet migrate back to the center and fuse, form in a diploid secondary nucleus. The three nuclei at the micropylar end from the egg –apparatus, of which the later ones are synergids and the median one is the egg-cell or the female gametes. The three at the chalazals end form the antipodals.
Fig. 3.8 b Development of Embryo-sac
In most plant the pollen tube comes into contact with the embryo-sac by way of the micropyle: this method of fertilization is known as Porogamy. When contact between the embryo-sac and the pollen tube is established otherwise, the fertilization is said to the aporogamous. Three types of aporogamy are recognized: (1) Chalazogamy, when the pollen tube enters through the funicle and chalaza; (2) Acrogamy, when the pollen tuble grows to the micropylar end of the sac but does not enter through the micropyle and 93) Mesogamy, when the pollen tube penetrates the ovule through the side.
Next to porogamy, chalazogamy may be said to be the most common. It was first discovered by Treub in 1891, in Causuarina. It is now established for many amentiferous genera like betula, Corylus, Fuglans, etc., and the families of Myricaceae and Fagaceae. Mesogramy had been reported in Populus( Poplar).
Fig. 3.9 a L.S of Ovary at fertilization
On reaching the embryo-sac, the pollen tube penetrates its membrane. Possibly the non-cellulosic nature of the embryo-sac proves helpful in the matter. The discharge of the male gametes may take place either by the dissolution of the wall of the pollen tube at its tip; or, the process may be facilitated by the synergids, through in what exact way is not quite clear. A noteworthy point is that the pollen tube, at no time penetrates the oosphere, even though fertilization of the egg-cell is the primary object of the entire process. The two male gametes move from the point of their liberation, one of the oosphere and the other to the secondary nucleus or to the pair of unfused polar nuclei at the center of the embryo-sac, as the case may be. Here, nuclear fusions follow. In this way, the act of double fertilization so characteristic of all angiosperms is fulfilled resulting in the formation of the zygote and the primary endosperm nucleus. (Fig.3.9 b).
Fig. 3.9 b Embryo-sac before fertilization
Several changes take place, in the wake of fertilization, in the flower. The externally visible of these are: (1) the sepals may or may not be shed; (2) the petals, the stamens, the styles and the stigmas dry up and fall away; (3) the ovary enlarges in size and becomes the fruit. However, most important of all is the transformation of the ovule into the seed.
3.10 DICOT EMBRYO DEVELOPMENT
Development of embryo in dicotyledonous plants. In many of the gymnosperms embryogeny begins with free nuelear divisions followed by wall formation. But the embryogeny ofangiosperms, in contrast, is like that of lower vascular plants. The first nuclear division of the zygote is followed by the formation of a cross wall, so that two cells are formed (ca and cb,) They contribute in varying degrees to the formation of suspensor and embryo proper.
The two celled proembryo is endoscopic in polarity. The lower r terminal cell forms the bulk of the embryo and the upper or basal cell present towards the micropyle forms the suspensor or hypocotyls-root of the embryo. The terminal cell of the 2-called proembryo may divide longitudinally (Crucifer and Asterad types) or may divide transversely (soland,d Caryophyllad and Chenopodiad types.
Crucifer types: In the year 1870 Hanstein traced the development of embryo in capsella bursa-pastoris and it represents a dicotyledonous embryo, the first division of the zygote is transverse forming a terminal cell (ca) and basal cell (cb). Then, the terminal cell divides vertically and the basal cell transversely resultingin a i-shaped, four called proembryo. Each of the two terminal cells divided vertically at right angies to the first, so as to result in quadrant stage. The quadrant cells divide in turn transversely forming the octants. Of these, the lower 4 cells are destined to form the cotyledons and stem tip and stem tip and the upper four to form the hypocotyls.
Fig 3.10 Dicot Embryo Development (Capsella Type)
Meanwhile, the upper two cells of the 4-called proembryo divide to form a row of 6-10 suspensor cells. Of these suspensor cells, the upper most one becomes vesicular and serves a haustorial function. The lowest cell of the suspensor functions as the hypophysis. It becomes somewhat rounded at the lower end and divides transversely. Each of the daughter cells divides twice vertically at right angles to one another forming two tiers of four cells. The lower tier of four cells contribute to the cortex of embryonic root the upper tier gives rise to the root cap and the adjacent root epidermis. Now the embryo consists f a filamentous suspensor and a spherical group of cells attached to it.
In the lower tier of embryo proper, further divisions take place at two points to form paired cotyledons as two ridges of tissue. The embryo now appears more or less cordate in longitudinal section. A few cells situated between cotyledon primordial constitute the future shoot the future shoot apex of the embryo. The hypocotyls as well as the colytedons elongate in size by transverse division of their cells. During later development, the ovule becomes curved like horse-shoe and the growing cotyledons conform to this shape.
UNIT – IV
Class – Chlorophyceae, Order – oedogoniales,
Family – Oedogoniaceae.
Occurrence: It is a freshwater filamentous alga. It grows epiphytically in ponds, pools and shallow tanks.
Morphology: The filaments of oedogonium are unbranched and consists of cylindrical cells except for the basal cell which is modified into a holdfast. Each cell is provided with a thickwall differentiated into three layers. The inner layer is made up of cellulose, the middle leyer is made up of pectin and the outer is chitinous- in nature. The terminal cells are rounded or acuminate and the intercalary cells often show an apical-basal polarity. Most of the cells at the distal end exhibit parallel striations or annular scars and are known as cap cells.
Fig 4.1 a Oedogonium Fig 4.1 b Oedogonium single cell
Cell structure: The cells are uninucleate and they have a central vacuole containing cell sap. Each cell has a recticulate chloroplast containing pyrenoids. A sheath of stach granules surrounds each pyrenoid.
Vegetative cell division. When the cell enters the division stage, the nucleus moves from the lateral position to the centre. Then a transverse ring of wall material appears on the inner face of the lateral wall just below the apical end of the cell. The nuclear division, the growth of the ring in thickness and the formation of a groove enclosing the growing ring occur concomitantly. Then a floating septum is formed between the two daughter nuclei.
Fig. 4.1 c Cell division and cap formation
The middle and outer wall layers external to the groove then rupture, permitting from elongation of the ring. This ring forms a new piece of cell wall lying between the cap and the sheath. Finally, the Floating septum moves upward and becomes fixed near the terminus of the old cell wall. Now the new cell has the wall formed from the thickened ring and the newly synthesized piece. The membranous striation of the ruptured parental wall at the anterior region of the upper daughter cell is the cap. The cell bearing it is known as capcell. The number of caps on a cell indicates the number of cell divisions that have taken place.
Vegetative: Vegetative reproduction by fragmentation is common. Under certain conditions Oedogonium may reproduce by Akinete formation.
Asexual: It occurs by multiflagellate zoospores produced within a cap cell. During the formation of zoospores the cell contents contract and semicircular hyaline area on one side of the protoplast is formed. A ring of basal granules appear at the base of the hyaline area from each basal granule a single flagellum is produced. Thus a ring of flagella around the base of the hyaline area is formed.
Fig 4.1d Oedogonoium – Zoopore formation
After the development of the zoospore, the cell wall near the cap region opens apart and the single zoospore moves out of the cell in a mucilaginous vesicle which soon gets dissolved thereby librating the zoospore. Thu only one zoospore is produced in a cap cell. Then the zoospare settlesdown on a suitable substratum. With its hyaline flagellar end. Afterwards the flagella are withdrawn and the zoospore becomes elongated the attached end of the zoospore differentiate into a hold fat.
Sexual Reproduction: It is advanced Oogamous. Oedogonium plant may be monoecius (both male and female reproductive organs occurring on the same plant) or dioecious (both kind of reproductive organs occurring on different plants.)
The male reproductive organs are called as antheridia and the female reproductive organs are called as Oogonia.
(A) Structure and development of antheridium: Oedogonium exhibits a remarkable variation regarding the development of antheridia. On the basis of the oedogonium has been divided in to two sub genera; (1) Macrandrous forms and (2) Nannandrous forms.
Macrandrous forms: The filaments of macrandrous forms are broader and in any cases are monoecious. Rarely they are dioecious. Antheridia the male sex organs are either terminal or intercalary. They are formed by the division of an antheridial mother cell. The division is similar to that of a vegetative cell, except that the upper cell, the antheridium proper is shorter than the lower cell or sister cell, because it does not elongate. The lower cell in this case divides repeatedly so as to give rise to a series of 2 to 40 antheridia. The protoplast of an antheridium may become metamorphosed into a single antherozoid in a few cases. But normally it divides transervsely or vertically and produces two antherozoids.
Antherozoids: The antherozoids are spherical and green in colour. They have sub-apical ring of short flagella at the base of colourless, beak-like anterior end. Their structure flagellation and method of liberation are similar to zoospore. But the antherzoids are smaller in size and have fewer flagella.
Fig 4.1e Oedogonium fertilization
In macrandrous dioecious forms oedogonium exhibits isomorphic dioecism, because of identify in both male and female plants.
2. Nannandrous forms: The nannandrous forms do not produce antheridia. But they develop andrsporangia.
(a) Development of androsporangium: Androsporangia develop just like the antheridia of macrandrous forms. The androsporangia may be developed on the same filament that bears Oogonia or in a separate filament. If Oogonia and androsporangia are present in the same plant, the condition is called gynandrosporous, But if both are on separate plants, the androsporangia bearing plant is called ididoandrosporous. The androspores are borne singly within theandrosporangia.
Fig 4.1f Oedogonium – Antheridia
(b) structure of androspores: The androspores are smaller than zoospores but the larger than antherozoids. Instead of being green, they are sometimes yellowish in colour due to the spresence of less amount of green pigments.
(c) Germination of androspores: Androspores are liberated; and at the end of a brief swarming period they settle down either on Oogonial wall or on one of the adjacent cells such as supporting cell. They germinate and produce plants of very minute size called as nannandria or dwarf males.
(d) The Nannandrium or dwarf male: It is produced by the germination of androspore It consists of only a few cells. It has a basal stalk cell or rhizoidal cell and above it one or two cells, the antheridia. Each such antheridium produces one or two antherozoids. They are liberated by an opening or a cap like lid. They swim about for sometime and enter an oogonium to fertilise the egg. These antherozoids are miniature (=small model) of antherozoids of macrandrous forms.
Fig. 4.1g A portion of filament with 2 dwarf males (DM) C-cap, R- Ring,
A – Antheridium
There is a remarkable dimorphism in the nannanorous forms. The male and female filaments can be recognized morphologically. The antheridia are produced on special, much reduced male filaments, the dwarf male male plants or nannandria. Thus, nannandrous forms show heteromorpoic dioecism
Fertilization: In both macrandrous and nannandrous species, the antherozoid enters into oogonia through the oogonial beak and fuses with egg or ovum. After fertilization oospore is formed which develop a thick-wall around itself. It is reddish in colour.
Germination of Oospore: The Oospore is set free from the filament by the disintegration of the oogonial wall. It undergoes a brieg period of rest. During germination, the oospore nucleus divides meiotically and thus four uninucleate zoospores are formed. They are made free by the rupture of oospore wall. Structurally, these zoomeisospores are similar to asexual zoospores. Of the four zoospores on germination two develop into male plants and two into female plants.
Diseases : Albugo or Cystopus causes white patches on the host and hence, it is commonly known as ‘white rust’’. All the species of Cystopus are obiligate parasites.
Albugo candida attacks all parts of the host plant causing white patches. Many important plants like radish, cabbage & turnip are commonly attacked by A. Candida. In A. candida, a number of forms which are morphologically identical differ in their ability to attack different crucifers. Thus, there are several biological strains.
Structure: Albugo is an intercellular parasite. It is commonly present in leves, but also attacks other parts of the host. The myceliumis very much branched. It is aseptate and multinucleate. The hyphae produce knob-like haustoria into the host cells to absorb food materials.
Reproduction: Albungo reproduces asexually by means of conidia. It can also reproduce by sexual reproduction.
Fig 4.2 a Albugo
Asexual reproduction: The hyphae grow vigorously and collect as a dense mat below the host epidermis. Then the mycelium produces many sporangiophores. They are unbranched and slightly thickened at the base. They are also called as conidiophores. The conidiophores are exposed when the epidermis bursts in moist weather. Each conidiophore has 6-10 nuclei and dense cytoplasm. The distal portion of the conidiophore is thin-walled. It is cut off as a single conidiosporangium orcondium by the formation of a septum below which it is rather gelatinous and it shrinks as conidia mature. Thus the conidia are cut off in basipetal succession. All these conidia are slightly separated from each other by slender connection called as disjunctors. The conidial chains, and the growing fungus exert pressure on the epidermis which bulges and then ruptures over the conidial sorus. The conidia will be released and they form a white crust on the host and hence, the name ‘’white rust’’. The conidia may be dispersed by wind or water.
In dry weather, the conidium directly germinates on the host producing a germ tube which penetrates into the tissues of the host. But in the presence of water, it produces 6-10 uninucleate biflagellate zoospores which are kidney shaped. These ones escape out into the water. Later, after a period of 2, 3 hours of swimming, each one encysts and produces the germ tube which infects the host through the stomata. When once infection is caused, the mycelium grows vigorously by producing haustoria into the cells of the host.
Sexual reproduction: At the end of the growing season of the host tissue, albugo reproduces sexually by sex organs.
The male sex organs are called as antheridia and the female sex organs are called as oogonia. The sex organs develop as terminal swellings of the branches of hyphae. The oogonia as well as the anheridia are formed at the apices of the branches and they are multinucleate. The antheridium will be close to the oogonium at its sides. The septum formed at the base of the oogonium or antheridium marks it off from the rest of the hyphae.
Oogonium: In the oogonium, many changes occur. As a result a dense central cytoplasm, consisting of the egg or ooplasm and a peripheral more vacuolated cytoplasm the periplasm are formed. But no wall formation takes place between the oosphere and periplasm.
In all species, the oognium well as antheridium are multinucleate at first. But species of Albugo differ in details of further progress of the sexual reproduction. On the basis of cytological details two types of fertilization can be observed. However fertilization in A.tragopogonis is explained here.
In this species, oogonium and the antheridium and multinucleats at the beginning. The antheridium is formed at the tip of another hypha and it will be lying against the oogonium on a side. The wall dissolves at the point of contact between the antheridium and oogonium. The content of the oogonium with an enveloping membrane bulges into the antheridium as a papilla. It was once thought that the papilla is meant for receiving the male contents and hence was named as respective papilla. But since it is withdrawn later its importance is uncertain. The antheridium produce a fertilization tube which grows through the periplasm and entersthe oosphere. The egg will become uninucleate at the time of fertilization due to degeneration of all the other nuclei. The functional female nucleus is close to he coenocentrum. Many male nuclei may enter the ooplasm from the antheridium. But only one of them fuses with the female nucleus resulting in a uninucleate oospore. After fertilization, the supernumerary nuclei of the periplasm and antheridium degenerate. A different condition exist in Albugo candida where uninucleate oospores are formed. However thecytological details are nuclei of the oogonium migrate into periplasm and only one remains in the egg. This nucleus as well as those of periplasm, divide mltotically once. But in the egg, of the two daughter nuclei formed, only one remains and the other degenerates . The coenocentrum is clearly distinguished. After fertilization, a uninucleate oospore is formed,
In A. Portulaceae multinucleate oospore is formed.
Germination of zygote: Mature oospore has a thick wall. The diplod nucleus divides repeatedly unitl it reaches a 32-nucleate stage. Meiosis occurs during this division. Inthis condition, it crosses over winter. In spring, crosses over winter.
In spring, mitotic divisions are resumed and a number of biflagellate zoospores(100-150) which are uninucleate are formed. Later on the oospore ruptures, so that the zoospores are liberated into a vesicle. Then the vesicle disappears; the zoospores swim out and then come to rest. They produce the germ tube which infects the host.
Habit and habitats: Funaria is a terrestrial moss which commonly grows in very dense patches of bright green colour. It usually grows in moist shady situations such as damp soil, shady banks, sometimes on the trunks of trees and on the walls. Funaria includes more than 177 species. Of these 15 species have been reported from India. Among these F.hygrometrica is the most widely distributed species which occurs through out the world. It usually grows in close tufts on the moist ground, sometimes on the walls, rocks or crevices. Frequently it occurs on recently burnt sites.
Fig 4.3a Funaria – Habit
The gametophytic phases of this moss consist of two growth stages namely. (i) Juvenile stages represented by the Protonema and (ii) the Leafy stage represented by the adult leafy gametophore.
(i) Juvenile Stage: It develops directly from the germinating meiospore. When fully grown it consists of a slender, green branching system of filaments called a protonema. The filamentous protonema branches freely. It forms a brilliant green coating on the damp soil resembling an algal growth. Many of the branches grow and spead over the moist soil. These are green chloronemal branches. The cells comprising them posses numerous, chloroplasts which are discoid and are separated by transverse cross walls. The other branches penetrate the soil, They are colourless or brown. The cells of these rhizoidal branches lack chloroplasts. They have oblique septa between them. The protonema stage in Funaria is only vegetative and transitory.
(ii) Leafy Stage: This stage starts as a lateral bud on the protonema. Several buds may develop on single protonema. Each of these develops into an erect leafy stem. The later bears numerous rhizoids at its base which anchor it to the substratum. Thus in many cases several leafy stems or shoots will be seen a rising from the same (protonema). At first they arise from all parts of the same individual (protonema). However the protonema soon degenerates and disappears. The upright leafy stems or shoots are left behind as separate, independent representatives of the gametophyte stage. Each of these is usually called the moss plant. It bears the sex organs.
External Features: The mature Funaria plant has a slender, upright, cental axis. It is about 13 min in height. It bears flat, green, lateral expansions inserted spirally. For convenience we call the central axis as stem and the green expansions as leaves.
Rhizoids: Apart from these two important organs of the moss gametophore, the third is the rhizoid system. It consists of numerous attaching filaments, the rhizoids which arise from the basal, naked, brown part of the stem. The rhizoids penetrate the substratum to a depth, at least, equal to the height of the leafy stem if not more. They are multicellular and branched. The main strand of the branched rhizoid which arise from the basal part of the moss stem is brown and stout. From it arise of the first order. They are colourless and the thin. The branches of the second order are of still finer caliber. When young the rhizoids contain minute oil droplets and become green when exposed to light.
The rhizoidal system is comparable to the root hairs of the root system. Like the root hairs their finer or ultimate branches are slender and colourless and have delicate cell walls. They are mainly absorptive in function like the root hairs. The older ones function in anchorage and conduction like a root system.
Stem: A C.S of the stem show epidermis, cortex and a central cylinder. The epidermis is uniseriate and stomata are entirely absent. The epidermal cells contain chloroplasts. The cortex is made up of parenchymatous cells in which, the outer cells have thicker cell walls. Chloroplasts may be present in the outer cells of the cortex. The central cylinder is formed of longitudinally elongated naroow, thin-walled colourless cells whick lack protoplasm and this region may serve conduction.
Fig 4.3 b funaria – T.S of stem
Leaf: the leaf lamina is one celled in thickness. But the midrib region is several-celled. The cells of the marginal part of the leaf contain many chloroplasts In the midrib region, the central narrow thin walled cells form a conducting strand. The cells outer to these small cells have thicker walls and they offer mechanical support.
Fig 4.3 c V.S of leaf
Sex organs: The sex organs do not arise singly but they are produced in groups, at the tips of the branches. Funaria is described as monoecious because the archegonia (female sex organs) and antheridia (male sex organs) arise on the same plant. The branches which bears male sex organs are called antheridial branch and the branches which bears female sex organs are called archegonial branches.
Antheridial branch: The male fertile branch has many antheridia enclosed by perichaetial or perigonial leaves which appear like a “floral envelope”. The whole structure resembles a flower and hence, called as ‘moss flower’. V.S. of a male branch shows antheridia of different stages of development, intermingled with sterile multicellular, uniseriate filamentous hairs, called peraphyses. The paraphyses have enlarged globular terminal cells.
Fig 4.3 d Funaria – V.S of Male branch
All the cells contain chloroplasts. The function of these hairs is not clearly known. But it is belived that they hold water by capillarity and prevent drying. The pericheaetial leaves are at the outer side forming an envelope around the sex organs. A mature antheridium is club-shaped and has a sterile jacket layer or wall of single-celled thickness, enclosing numerous fertile spermatogenous cells or spermatocytes. The protoplast of each spermatocyte metamorphoses into a spermatozoid or antherozoid. Each sperm is an elongated, coiled, biciliate structure with a single nucleus and cytoplasm. A mature antheridium shows at the apex one or two cells of the jaket layer Distinguished into an operculum. At maturity the antheridium absorbs sufficient water, as a result, the contents antheridium absorb sufficient water, as a result, the contents swell up and exert pressure, bursting the cells of the operculum. The sperms are forced out through the small opening formed at the top.
Archegonial branch: The archegonia are the female sex organs. They are produced at the tips of the lateral branches. A longitudinal section of the apex shows archegonia, with paraphyses in between.
Fig. 4.3 e Funaria – V.S of Female branch
A mature archegonium is a flask-shaped structure. It has a massive columnar stalk. The basal enlarged part of the archegonium is the venter. It has two celled thick wall. The venter has the egg-and ventral canal cell. The archegonium also has a long narrow neck, the wall of which is one-celled in thickness. The neck contains a central row of 6-9 neck canal cells. At the time of fertilization all the cells, except the egg disintegrate and come out as a sugary substance which attracts the androcytes or sperms.
Fertilization. The sperm fertilizes the egg. The oospore later on develops even while it is in the venter and produces the capsule.
Structure of the capsule: The structure of the capsule can be best understood by studying the same in L.S.
The capsule has three important parts 1. A sterile part incontact with the seta, called the apophysis, An upper fertile part or theca producing the sporogenous layers and 3.An apical portion differentiated into an operculum covering the peristome.
Fig. 4.3f Funaria V.S of capsule
Apophysis: The apophysis has a distinct epidermis with stomata. These stomata facilitates the free exchange of gases. Inner to the epidermis, the parenchymatous cells contain many chloroplasts so that the tissue is photosynthetic in function. This shows that the sporophyte is only partially dependent on the gametophyte. In the centre of the apophysis region there are elongated, narrow parenchymatous cells. They are continuous with the conducting strand of the seta.
Fertile region: In the upper fertile portion of the capsule below the epidermis, there are two or three layers of colourless cells, forming the hypodermis. This hypodermis is thin towards the lower side. The parenchymatous cells, inner to the hypodermis, contain chloroplasts. Some of these cells from green filamentous strand or trabeculae. These filament extend across the air space or lacuna connecting the outer spore sac with parenchyma. The central sterile mass of parenchymatous cells constitute the columella. It is surrounded by the fertile tissue known as the archesporium. It has the spore mother cells. The archesporium is surrounded on the outer and innersides by the spore ssac which supplies necessary food materials tot the spore mother cells during development. The outer spore sac is about three layers in thickness and the inner spore sac is single-layered.
Operculum: The apical part of the capsule has a caplike structure known as operculum. At the base of the operculum, some specially thickened cells are present inring called and thin-walled and they break up so that the operculum is shed at the time of dispersal of spores just below a the operculuma rim is present just below the operculum.
The operculum or lid covers the peristomial teeth. The peristomial teeth are in two rows. Each row consists of 11 peristomial teeth. The outer ring of teeth have thick transverse bands The peristomial teeth help in the dispersal or spores.
Spore: When the capsule dehisce the spores are scattered by the wind. They germinate under favourable conditions. The spores grow into a green filamentous structure known as protonema. The protonema produce many rhizoids. It also produces many lateral buds which grow up into new moss plants. Thus the life-cycle of moss is completed.
Fig 4.3g Porotonema
4.4 LYCOPODIUM (the club moss)
Lycopodium is a large genus with 200 species, commonly called ‘club-mosses’. It is a world-wide in distribution, through chiefly found in the tropical and sub-tropical forests. About 30 species were reported from India by Chowdhury (19370, Some of the common Indian species are: l.cernum. L.Phlegmaria, L.clavatum, l. wightianum. Many species prefer moist and shady situations and few species like L.phlegmaria, are epiphytes.
SPOROPHYTE (external features)
They have small, herbaceous sporophytic bodies, living epiphytically or terrestrially. The stem may be erect of horizontal, above or below the surface of the soil and is densely covered with small leaves The plant body is branched dichotomously. The two branches may grow equally well or one of them may grow better than the other resulting in a monopodial like branching.
Leaves: The leaves are simple and ressile small and eligulate, 2 to 10mm. Long, simple and numerous, covering the stem closely. They are arranged inclose spirals (L. clavatum)or whorls (L. verticillatum) or opposite pairs. (l.alpinum) or the arrangement may be irregular. A single medien vein is present. The leaves are heterophyllous in somespecies (L. volubile).
Roots: In the older plant the roots are adventitious arising singly or in groups acropetally along the underside of the stem. They are some what irregularly distributed. But in some species, the roots branch typically dichotomously, in some other species the dichotomous nature of branching is not clear.
Anatomy of the stem: T.S. of the stem of Lycopodium shows an epidermis, a broad cortex, and a central stele.
The epidermis is one cell in thickness with thick cutinized outer wall and is interpreted by stomata. The nature of cortex varies from species to species. In some species it is as broad as the stele, in others its radial thickness is several times larger than that of the stele. In some species, the cortex is wholly made up of parenchyma (L. inundatum) and therefore soft; in other species either the inner or the outer portion of the cortex is sclerified (L. clavatm) in still others, the entire cortex may be sclerified.
Fig 4.4 b V.S of Lycopodium stem
In some species, at least in young portions, an endodermis can be observed. Internal to the endodermis is a pericyle of three to six layers of cells in thickness.
There is much variation in the arrangement of the vascular elements not only in different species but also within a single plant body! On the basis of the arrangement of vascular elements different steles are recognized:
1. Protostele: In the sporelings the vascular element form a solid axial cylinder in which the central core of xylem is surrounded by the phloem. This type of stele is known as protostele.
2. Actinostele: This is a modified rayed protostele where the xylem-core has got radiating ribs forming a star-like mass in cross section. The few protoxylem strands 3 to 12 in number are exarch andsituated at the periphery of the ribs. Eg. L.phlegmaria L serratum. In L.serratum the xylem are expanded outwards into a wide-spread structure with a fan-like outline in cross section.
3. Plectostele: In some species the solid xylem core becomes grooved by the interpolation of softer tissues, and the furrows in the xylem cylinder become numerous and less regular. In C.s. the xylem appears as a large number of isolated strands. Such stems may have the xylem-core in the form of plate-like lobes (as seen in cross section) andxylem and pholoem symmetrically arranged in alternating transeverse bands across the stele, e.g., L. volubire. L. wightianum etc. such parallel banded type of ptotostele are knows as plectostele.
Fig 4.4c Plectostele (X-xylem, P-Phloem)
4. Mixed protostele: In L. cernum the stele may have the mass of xylem and phloem more uniformly distributed, the former as irregular scattered groups embedded in the ground mass of the later, as seen in C.S Such a stele is known as mixed protostele.
Fig 4.4d Mixed Protostele (X-xylem, P-Phloem)
ANATOMY OF THE ROOT: Anatomically the roots of Lycopodium exhibit a differentiation in epidermiss, cortex and stele. The cortex is made up of many layers of cells. The stele is is either monarch (with one protoxylem mass) or diarch (with two protoxylem points) or triarch( with three protoxylem points) very often, the setele is diarch with C or u-shaped xylem, the opening of the C or U facing away from the stem. The protoxylem lies at the tips of this xylem mass, with intervening portions made up of metaxylem. The phloem lies betweenthe points of the C or U.
During reproduction, some of the leaves develop sporangia on their upper side near the base or even on the stem just above the leaf. These fertile leaves, are called as Sporophylls. In some species they are smaller; in still other species, they differ greatly from the ordinary leaves in size, shape and colour. In some species these cones are sessile on leafy stems, the foliage leaves and the sporophylls being connected by transitional forms, In other species the cones are borne on erect stalks with distinct, scale, leaves. In a few species, every mature leaf bears sporangiaand in such forms definite cones are lacking. In L. Selago the vegetative leaves and sporophylls alternate with each other. It is called as ‘selago’ condition.
Sporangia: The sporangia are large, ranging from1 to 2-2.5 mm in diameter, kidney-shaped or subsperical. They are yellowish in colour and stalked or may have only a pad-like base. In those species where the sporophylls are aggregated into cones the sporangia may be wholly covered by the dorsal faces of the sporophylls above. They open a slit transverse to the leaf, two clam-shell like valves being formed. The sporesare dispersed by wind after liberation.
Fig 4.4e V.S of Lycopodium cone Fig 4.4 f Lycopodium sporangia
The Sporangium develops from Sporangial initials. They undergo periclinal divisions. As a result an outer and an inner layer of cells are formed. The out layers later on produce wall layers and tapetum. The inner layer produces Sporemother cells. The Spore mother cells undergo meiosis and produce Spores.
Spores: The Spores are very small, light and thin-walled. The walls are smooth or pitted which honey comb or network-like thickening.
Germination of spore: The spore germinates in to a prothallus. The prothalli of Lycopodium are of two kinds.
The first type is green (except at the base) and grows on the surface of the ground. It is cylindrical or ovoid with lobed or branching top. It is small-sized, only 2 or 3 mm long. It matures quickly and is short-lived.
The other type is non-green. Subterranean tuberous and large, 1-2 cms. Long or wide. It has a top-like or carrot-like shape, but sometimes it resembles a disc or kernal or maize. The prothalli complete their development over very long periods; in some species the spores require 3 to 5 years and in other 6 to 8 years for germination and may require another 6 to 15 years to mature. They live for several years after maturity, nourishing the young sporophtes for many years. The first type of the prothallus i.e., the short-lived, flat type of the prothallus i.3., the short-lived, flat type is found in the tropical species like L cernum and the second type is characteristic of the northern species like L. complanatum. L.clavatum and also of epiphpytic forms. In both the types there is an endophytic fungus in association with the tissues but in the subterranean type, this mycorrhizal conditionis a very prominent feature.
In L. Complanatum the prothallus gives a fairly good idea of the structure of the prothallus of its type. In this type the surface is covered by an epidermis, which is followed by a cortex of several layers. Inside the cortex is a palisade tissue composed of a single layer of elongated cells. The outer cells contain the mycorrhizal fungus forming the so-called hyphal tissue. There is a central mass of hexagonal cells which store food. All these tissues form a wedge-shapped structure with a more or less flat top. The core of the wedge and the irregular upper face less flat top. The core of the wedge and the irregular upper face are parenchymatous. a more or less flat top. The core of the wedge and the irregular upper face less flat top. The core of the wedge and the irregular upper face are parenchymatous. The entire top surface is developed to the production of sex organs and is regarded as the generative tissue. Its cells do not have any food reserves and for some remin meristematic. The antheridia are produced at the centre of this generative tissue and archegonia are produced towards its outer rim.
Antheridium: It is the male sex organ which form a single superficial cell of the prothallus. This divides into an outer and an inner cell. The outer cell produces the covering layer of the antheridium and the inner cell produces sperm mother cells. The mature antheridium consists of an oval mass of antherozoid mother cells slightly projecting from the prothallus and surrounded by a wall, which is derived partly from the covering layer and partly from the prothallial cells. The sperms are fusiform are biflagellate at the anterior end.
Archegonium: It is the female sex organ. It develops on the same prothallus as the antheridia but towards the margins .Usually many archegonia are formed but only one develops after fertilization. The archegonium, arises from a superficial initial which divides again giving rise to a primary canal cell transversely to form four to eight neck canal cells. Whether the primary ventral cell directly serves as the egg cell or it undergoes a division giving rise to a ventral canal cell and an egg has not been fully established.
Fertilization: Fertilization is not reported so far but a disintegration of the axial cells except the egg occurs. The opening of the archegonium seems to be brought about by the spreading apart of the neck cells as also by the disintegration of the uppermost neck cells.
The Embryo: The zygote cell first undergoes a transverse division (transverse to the long axis of the archegonium.) The upper cell forms a suspensor which does not take any further part in the embryonic development. It pushes the other embryonic cell down and enables it to develop and intimate association with the prothallial tissue. The embryonic cell divides giving rise to an octant in which the cells are in two super-imposed tiers of four cells each. The four upper cells of the octant develop to produce a tuberous mass of cells of the octant develop to produce a tuberous mass of cells called the foot. The foot remains as an intra-prothallial) haustorium. The haustorim helps to draw nourishment from the prothallus until the embryo is independent. Of the four cells at the lower end in the octant, two form the stem and two form the first leaf. The first root develops exogenously near the base of the first leaf. Subsequent roots are endogenous. As the stem grows it emerges from the prothallus and more leaves are formed. The first leaves do not have vascular supply and are known as prophylls. Ultimately the prothallus decays and the young embryo develops root and becomes fully independent.
Protocorm: The description given above is true for all those species with subterranean prothalli. But in the species with surface prothalli the zygote develops into what is known as protocorm. Its development also proceeds the same way as the embryo up to the octant stage. The foot is small and serves little as an absorbing organ. The other cells, by further divisions give rise to a parenchymatous spherical mass, the protocorm. It is provided with root hairs and has on its upper side a few cylindrical green leaf-like projections. The protocorm is also in association with a fungus the prothallus.
4.5 CYCAS (20 SP.)
Class: Cycadopsida, order : Cycadales, Family: Cycadaceae Distribution: The genus cycas is represented in India by a few species, of which C. pectinata,
C. rumphii, C.circinalis are common. C. revolute and C.rumphii are widely cultivated in India gardens some species yield a kind of sago. So Cycas is also called as ‘’sago palm’’.
Morphology: the sporophyte of cycas bears very close resemblance to the date palm. Cycas has a stem. It is columnar and unbrached, usually ranging from 3 to t5 meters in hight. The thich trunk is covered by an armour of persistent leaf bases and it bears a crown of large pinnate leaves at the top.
Fig 4.5 a Cycas habit
Leaves: In cycas, there are two types of leaves namely the large foliage leaves and scale leaves. These are arranged spirally at the top. The foliage leaves are large, and pinnately compound, sand about 1-3 meter length. They are attached to the stem by expanded leaf bases, The petiole is short and the long stout rachis bears numerous, thick, leathery green leaflets in two rows towards the adaxial side. They have midrib only any lateral veins are absent. In young leaves, the rachis as well as the leaflect are circinately coiled like those of the ferns. The scale leaves are brown and leathery.
Rootsystem: The tap root is quite thick and bears lateral roots. Some of the branches of the lateral roots come just above the soil and become aerial. These are negatively geotropic roots and they are dichotomously branched. The coralloid masses of rots are called as coralloid roots.
Coralloid roots: The coralloid roots are formed at the surface of tdhe soil. Bacteria of the soil enter and rapidly multiply in the cells of the apical portion. Consequently, the cells disorgnise and the intercellular spaces are enlarged. Then, alagae like Anabaena and nostoc enter into the disorganized zone of the cortex forming an algal zone. Coralloid roots are much branched and resemble the coralloid masses and that is why they are known as coralloid roots.
A T.S. of coralloid root shows epidermis as the outermost layer. But cork may be formed at the surface in oder roots due to the phellogen which will not be distincti. This is followed by cortex and stele. The cortex is distinguished into three zones. The outer cortex is composed of parenchymatous cells and the middle cortex shows Anabaena andprobably also nostoc filaments. The inner cortex is diarch or triarch or tetrarch and the xylem is typically exarch. Thus, the coralloid root differs from the normal root in having poorly-developed vascular tissues or more and, in possessing a much wider cortex with a median algal zone. There is no root cap.
Fig 4.5 b T.S of Coralloid root
Anatomy of Stem: A T.S of stem shows a large parenchmatous cortex and pith scanty vascular tissues in between. The epidermis is obliterated by the occurrence of persistent leaf-bases. The parenchymatous cells of the cortex are thin-walled and they contain a large amount of starch as reserve because they are a source for the extraction of “sago” in Japan and China. The cortex, which is outside the vascular ring, and the pith are connected by the medullary rays. Mucilage canals are present in the cortex and pith region. The vascular bundles are collateral, endarch open and are arranged in a ring. The cambium is present. The phloem consists of sieve absent instead in the phloem, albuminous cells are present.
Fig 4.5 c T.S of Cycas stem Fig 4.5 d T.S of Cycas old stem showing
successive vascular element.
Secondary growth: Secondary thickening occurs due to the activity of cambium ring. The primary cambium is active for a short period only and secondary growth takes place by successive rings of secondary cambia. The secondary rings originate from pericycle (Jeffrey). The amount of secondary vascular tissues formed is small and so the stem is mostly parenchymatous. The wood is traversed by numerous broad parenchymatous rays. This type of loose-texured wood of cycas is known as manoxylic in contrast to the dense massive wood of coniferales which is described as phyconoxylic. Periderm is formed at the surface.
Leaf traces: The leaf traces of cycas are peculiar in many ways. The leaf trace arises from the stele on the side opposite to the insertion of the leaf. Soon after emerging, it divides into two branches which encircle the stele in opposite directions before entering together into the petiole. Such a leaf trace is known as girdling leaf-trace. In the rachis, they divide further into numerous bundles. Apart from these traces each leaf receives direct leaf traces also.
Fig 4.5 e Young normal root Fig 4.5 f C.S of Cycas leaf
Cycas reproduces vegetatively by means of adventitious buds or bulbils which are produced all over the stem from the apex to the base, in the crevices of the lower fleshy portions of old leaf-bases. They germinate under favourable condition to produce a new plant after falling on the soil.
In Cycas circinalis, the production of new plants from suckers which arise from profusely branched horizontal root system has been reported.
Sexual Reproduction: Cycas is dioecious. Young plants do not bear any cones which appear only after the plants are about ten years old.
Male strobilus: The male cones are very large and they measure 80 cm. Or more.
The male cone (staminate strobilus) is produced singly at the apex of the stem. The first cone is terminal, so the apical growth of the stem is stopped. Later on, however, the cone is pushed on one side and the stem continues growing by lateral bud. Thus the stem in male plant is described as ‘sympodial’.
Each male cone is a long, compact, fusiform or roval structure, measuring 40-60 cm in length. It consists of numerous microsporophylls or stamens arranged spirally in acropetal succession. They are soft and fleshy at first later on become hard and woody. The large cone may consequently shed its pollen from the topmost sporophylls several days before the lower ones dehisce. Just before shedding the pollens, the cone elongates rapidly and the sporophylls get separated from each other. They become hard and dry on account of loss of turgidityand become loose. These change in the old cone facilitate dispersal of pollen-grains (microspores) from the pollen sacs (microsporangia) which have by now become almost exposed.
Fig 4.5 h Cycas : Male cone Fig 4.5 i Microsoporophyll of cycas
1. Entire 2. L.S of cone 1. Sori. 2. Point of attachment.
Microsporophyll or stamen: In Cycas circinalis, it measures 3-5 cm, in length. It is a flattened, some what triangular structure narrow below and broadened above into an expanded sterile disc drawn out into a sort of projection, the appophysis. It shows no trace of the pinnate character of the vegetative leaves. On the under surface or the abaxisal side are borne numerous, rarely few pollen sacs( micro-sporangia) which are scattered or arranged in groups or sori or from 2 to 6 around a minute central protuberance. They dehisce by a silt which extends radially from the centre of a sorus. The numbers of pollen sacs (microsporangia ) on a single microsporophyll varey from 1000 in Cycas up to 700 in C. circinalis. The sporophylls near the top and the bottom of the cone have fewer sporangia, so much that more reduced sporphyllys at the extreme top and the bottomof the cone are entirely sterile and bear no sporangia at all.
Dehiscence of the Sporangium: When the spore mother cells enlarge and divide, the tapetum breaks down and appears, as a mass of nucleated protoplasm surrounding the spore mother cells, which absorb not only the tapetum but also the wall cells between the tapetum and epidemis. Some of the cells of the sporangial wall at the anterior end modified and behave like the annulus of a fern. It is at this point that the sporangium dehisces on maturity. So as to allow the pollen-grains to escape.
The Female cone: Cycas is peculiar in its ovulate strobilus in that, it is not a true cone or strobilus but simply a rosette of megasporophylls, arising spirally in acropettal succession. They are loosely arranged on the stem in a rosette, like ordinary crown of foliage leaves around the stem tip. The megasporophylls, like the crowns of foliage leaves are produced only once in year. The megasporophyll elongates and pushes through them, they assume an horizontal or drooping position.
Megasporophy II : It has three well defined parts: the petiole, a middle stalk- like portion bearing variable number of ovules and a distal or upper sterile part which is variously lobed or serrated. It is 6 to 8 inches long and resembles a reduced foliage leaf. There are usually three pairs of red ovules, sometimes two and rarely four pairs attached laterally on the unbranched stalked portion of the sporophyl and apparently replace the pinnae on this part of the sporophyll. Both the ovules and sporophylls are covered by a thick coat of yellow woolly hairs.
Fig 4.5 j Cycas female sporophyll
Megasporangium or Ovule: The ovules are erect (orthotropous) andvery large about 6-7 cms in tength. The ovule has nucellus covered by integuments.
Integument: The integument is very thick. In a matute ovule, it is differentiated into three layers-an outer fleshy layer of green or red colour, the middle stony layer and an inner fleshy layer. Although three-layered, the integument seems to be all but one structure. Usually the ovules are suppied with two vascular strands from the megasporophyll. The outer strand enters the outer fleshy layer at the second strand traverses the inner fleshy layers,
Development of Megaspore (Embryo sc cell): At the beginning all the cells of the nucellus are alike. Megaspore mother cell arises deep within the nucellus. Itdiffers from the neighbouring cells in its large size denser contents, and conspicuous nucellus. It divides twice and forms a linear tetrad of four megaspores. The upper three cells are small and do not develop further. The lowest is the largest and grows at the expense of the smaller three. This forms the sngle functional megaspore or embryo sac cell. As it enlarges the surrounding cells of the nucellus take on the charcter of’ spongy tissue’ as in pinus.
Germination of pollen-grain: The pollen-grain is the first cell of the male gametophyte. It has two walls, an outer exine and inner intine. It is boat-shaped andbears a longitudinal slit or furrow. During its maturationon the tenth day a large vacuole appears in the cytoplasm. As a result the nuclus is shifted to one side. It starts germination while still in the microsporangium (pollen sac). The vacuole disappears before it stars its division, which takes place when it is about three weeks old. At first, a small vegetative cell or prothallial cell is cut of at on end, leaving a large antheridial cell. The vegetative cell represents the male gametophyte proper, which is very much reduced and completely dependent. The antheridial cell soon undergoes transverse division so as to produce a smaller generative cell in contact with the prothallial cell and a large tube cell with a comparatively larger nucleus than the generative cell. At this three-celled stage the microspore is shed from the microsporangium.
Pollination: The pollen is light and dry and easily carried away by wind in three-celled stage. At the time of pollination, a large “pollination drop” or mucilage oozes out at the micropylar end of the ovule. The pollen grains fall directly on the drop. As the drop dries up, the pollens are drawn into the pollen chamber below. Further drying seales the chamber top & becomes very hard. The apex of the nucellus may grow out into a beak-like structure called the nucellar break, which projects into the micropylar canal.
Post-pollination chages inpollen-grain (Microspore): After pollination, the microspore undergoes the following changes:
1. The tube cell elongates and pierces through the exine forming a long branched pollen tube. The nucellus of the pollen tube remains in the centre or moves to the branches. The pollen tube in Cycas acts as an haustorium and penetrates slowly through the nucellus for a few months, gradually digesting and absorbing it. Thus it acts as an haustorium or absorbing organ besides acting as a sperm-carrier as in other gymnosperms.
2. By this time, the generative cell becomes large and divides into two, the stalk cell and the body cell.
3. There is no further division for a long time s the body cell divides only just before fertilization.
4. Interval between pollination and fertilization is about four months in Cycas revolute. During this period the pollen tube eats its way downward, until it finally completely pierces tube eats its way downward, until it finally completely pierces through the nucellus and hangs freely in cavity which is partly pollen chamber and partly archegonial chamber. Further partly pollen chamber and partly archegonial chamber. Further development is confined to the body cell; however, enlarges considerably and becomes filled with starch and other nutritive materials. At the same time the prothallial cell penetrates the stalk cell, reducing it to the form of a life belt like ring.
5. As the body cell enlarges, structures called blepheroplasts consisting of a number of fibres adiating out from a conspicuous centre, appear one at each pole of the nucellus. These blepheroplasts are concerned in the production of cilia.
6. The body cell divides into (just before fertilization,) one blepherolast passing into each daughter cell. The two daughter cells function as sperem mother cells. Each producing a sperm. The body of the sperm is produced from the nucellus, while the blepheroplast produces thousands of cilia.
Sperms of Cycas: The sperms of cycas are remarkably large. They are visible to the naked eye as top-shaped, oval structures. These are naked behind and spirally coiled in the anterior half with many short cilia emerging from the grooves. The sperm begin to move while still in the sperm mother cells. The sperm mother cell, soon break and the sperms come to lie freely in the cavity of the body cell. They swim in the cavityof the body cell for about half an hour, before they separate and then pass into pollen tube. Then they reach the tip of the turgid pollen tube to be discharged during fertilization.
Germination of Megaspore (Embryo sac cell)
The megaspore is the first cell of female gametophyte. It enlarges considerably by absorbing some of the neighboring cells, before it divides. By the time the first divisionof the nucleus has been completed, there is a great change in the surrounding cell, which form a layer called the “spongytissue”. The megaspore nucleus now divides repeatedly by free nuclear division, forming 1,000 or more nuclei which lie freely in the cytoplasm. Later on, vacuole, appears in the centre and the nuclei embedded in a thin layer of cytoplasm are shifted to the periphery. Later a parietal layer of cells is formed by free cell formation near the periphery; cell walls are formed and a parietal tissue developed which continues to grow towards the centre, This results in the formationof a female prothallus cavity of the embryo sac: This is small-celled endosperm, filling the celled below. Lower cells contain a large amount of nutritive substances, thereby providing food for the embryo it is not green and therefore, totally dependent on the food supplied by sporphyte. But in Cycas circinalis, it is reported that when fertilization fails, the endosperm may protrude out of the micropyle and turn greenon exposure to light. Long before the gametophyte is actually formed a nutritive layer 1to 2 cells thick is developed outside it. This is called the endosperm jacket spongy tissue. The cells of the spongy tissue invade upon the nucellus, whose cells absorbed thus providing nourishment to the germinating megaspore orthe developing female gametophyte.
Structure of Archegonium: Each archegonium consists of two cells a venter canal cell represented by an evanescent nucleus which usually disorganizes and an egg. The evanescent nucleus representing the venter canal appears to have no function, but only represent the survival of an ancestral character. There are no neck canal cells and the venter which is not distinct, is formed by the surrounding cells of the female prothallus.
Fertilization: At first the end of the pollen tube, containing tube nucleus and the sperms grows toward the embryo sac. The turgid end of the pollen tube bursts and discharges its contents in the liquid of the chamber above archegonia, towards the necks of which the sperms swim and make their way down to the egg cell. As the sperm passes into the cytoplasm of the egg, its own cytoplasmic membrane along with the cilia slips off, while the nucleus fuses with the female nucleus, forming the oospore.
Embryology: At first the oospore develops into a proembryo. The oospore does not undergo any period of rest. It contains a large nucleus which divides repeatedly by free nuclear division. This results in the formationof numerous nuclei soon after fertilization, which are distributed throughout the cytoplasm of the oospore. Coulter and cyamberlain mention that “there are certainly as many as 256 nucleiin C. revolute and very probably more”. In C.Circinalist, however as reported by Rao the number of nuclei is more than 128, but never 256. They are later onpushed towards the periphery of the egg due to the appearance of a central vacuole. Later onwalls are formed between the nuclei, resulting in the formationof tissue which fills up the whole of the vacuole. This is known as the fills up the whole of the vacuole. This is known as the proembryo. Even where persistent cells walls appear above, the upper portion simply functions as a large food reervior, contributingno cells to the formation of the embryo or new plant.
The proembryo soon becomes differentiated into three regions-an upper haustorial portion is in contact with the nutritive material above, a middle zone of elongating cells forming the suspensor and a terminal group of cells constituting the embryo itself.
The cells of the suspensor continue to elongate till they from an exceedingly long, spirally coiled suspensor (some times 5cm.or more long.) It helps to force the embryo out of the archegonium into the endosperm. At a time all the archegonia may be fertilized and each oospore may produce a suspensor. All these suspensor twist together and form a compound structure supporting the one embryo that develops to maturity. The terminal cell at the end of the long suspensor forms the embryo which consists of redicle, plumule and cotyledons. As there are several archegonia we may find several embroyo in one seed. It is known s polyembryony. But only one develops out of all these and the other perish.
Seed formation: The following important changes occur during seed formation.
1. The integument on hardening becomes the seed coat which consists of an outer fleshy layer, middle stony layer and the linner fleshy layer
2. The nucellus is completely curshed and used up in the nourishment of the embryo.
3. The inner flesly layer of the integument is also absorbed partially.
4. The embryo is straight, attached to the long coiled suspensor and embedded in fleshy endosperm
Embryo: The matured embryo has a short axis, the hypocotyls, terminating at the end next to the suspensor in a root tip or radicle. This is enclosed in a hardcovering, the coleorhizha. At the opposite end of the hypocotyls is a minute stem tip, the plumulue, lying between pair of seed leaves orcotyledons. The cotyledons are sometimes unequal. Where only one cotyledon is normally present, the second is lateral. The ripe seed is usually cream-coloured, orange or red. According to Rao, the testa has a sweet taste and pleasant odour, which helps in their dispersal by animals and birds.
Germination of seed: The cycas seeds germinate promptly without undergoing a period of rest. But according to De Silva and Tambiah, in C.rumphii the seeds undergo a long period of rest. During germination, the radicle pierces through the coleorhiza, grows down and formsa strong tap root. Later on the coleorhizadries up and become papery. The cotyledons remain embeddedin the endospermandbeing haustorial, they gradually absorb the food stored in the endosperm and pass on the nutrient to thedeveloping seedling. Later when the food (endosperm) is exhausted, the shriveled cotyledons still enclosed in the seed coat become exposed at their base, hence the seed germination may be said to be epigeal. The plumule grows out between the cotyledons and becomes erect. The shriveled cotyledons still anchored in the testa of the seed may continue to remain attached in the testa of the seed may continue to remain attached to the young plant for a year or more. The first one of few leaves are generally scale leaves, followed by a pinnate foliage leaf with a few leaflets.
Fig 4.5 l Cycas : Proembryo Fig 4.5 m Cycas : Sedling and portion of leaf showing
The young leaf shows circinate vernation. The seedling stem remains substerranean and insignificant for several years, until it comes out and becomes colummar. The first crown of leaves appear much later, usually several years after the formation of young seedling. The earlist leaves are invariably of the nature of scales.
UNIT – V
Osmosis can be defined as a special case of diffusion in which only solvent but not solute molecules diffuse when a solution is separated from its own solvent or a dilute solutions is separated from a concentrated solution by a semipermeable membrane.
Thistle Funnel Experiment:
A thistle funnel is taken and over its mouth is spread tight and tied a sheep’s bladder. Inside the funnel is kept a strong solution of sugar and the mouth of the funnel is supped into a beaker of water. The initial level of the sugar solution is marked on the stem of the funnel. Very soon, there is a rise in the level of the solution in the funnel, showing thereby that the water from outside has entered the funnel through the animal membrane. This happens because the concentration of water in the beaker is greater than that in the funnel. Water from the beaker centers the funnel through the membrane to equalize the concentration on both sides. The sugar, however, is no capable of passing through the membrane as freely as water and mix with water in the beaker. Since the membrane permits the free passage of water but not of sugar, it is said to be a semi-permeable membrane. The passage of the solvent through such a membrane is called osmosis. It is by osmosis that root hairs absorb water from the soil.
Fig 5.1a Experiment to demonstrate osmosis
The root hair may be compared to the thistle funnel in the experiment. The sugar solution corresponds to the cell sap, the animal membrane to the cytoplasmic layer and the water, in the beaker to the soil water. So, conditions are quite favourable for osmosis. Since the soil water is only a very thin solution, the proportion of water in it is greater than that in the cell sap. In other words, the concentration of water is greater outside than inside the root hair. So, water enters the root hair. The concentration of the salts in the soil water is greater than that in the cell sap and in order to equalize the concentration, they too enter the root hair along with the water. But the entry of water is independent of the entry of mineral salts. The entry, however, is always from a region of higher concentration to one of lower concentration.
5.2 ABSORPTION OF WATER
The absorption of water from the soil takes place chiefly through the root hairs. The root hairs are in contact with the water films on the soil particles. Inside the root hair is a thin lining of cytoplasm which encloses a large vacuole filled with cell sap. The cytoplasm and cell sap of the root hair are continuous with those of the root cell of which it is a prolongation. The cell wall of the root hair is a permeable membrane. Its cytoplasmic lining is a semipermeable membrane. The cell sap is an aqueous solution of mineral salts, sugars and organic acids. As a result it exerts high osmotic pressure (3-8). The soil solution is relatively dilute and has a low osmotic pressure (less than 1atm). The cell wall of the root hair imbibes soil water as it contains pectic substances. From this point water passes into the root hair across the semipermeable cytoplasmic membrane by osmosis on account of the higher osmotic pressure of the cell sap.
Fig 5.2 a Absorption of water
The root hairs are merely outgrowths of the outermost cells of the root. The root hair cells are in contact with the cortical cells which extend to the endodermis. Internal to the endodermis is a single slayer of parenchymatous pericycle which at contain points lies opposite to the protoxylem elements of the xylem (Fig.6.5). The arrangement offers a direct channel for the passage of water to the xylem at these points.
The absorption of water from the soil the root hair cell a (Fig. 6.5) becomes fully turgid, its osmotic pressure falls due to dilution and its turgor pressure increases. As a consequence its suction pressure will fall below that of the adjacent cortical cell b, with the result that water will pass from a to b. The diffusion of water into b likewise reduces its suction pressure which falls below that of the next cortical cell c, with the result that water passes from b to c. In the same manner water passes from the cell c to d, from d to e from e and f and from there into the endodermal cell g. From here it is passed on to the pericyle cell h which will eventually become turgid. It will then exert no suction pressure and hence will readily give up water to the xylem vessel i with which it is in contact. The walls of the xylem vessels are inelastic so that there is no turgor pressure and the whole of the osmotic pressures of the xylem s ap constitutes its suction pressure. This being higher than the reduced suction pressure of the parenchyma (pericycle) cell h, water will be drawn into the xylem vessel. It will be clear that the factor which determines the movement of water from the soil to the xylem via the root hairs and cortical cells is the existence of what may be called a gradient of sution pressure (diffusion pressure deficit) from the root hair to the xylem vessel.
According to the mechanism of water absorption outlined above, water is forced into the xylem vessels by the surrounding cortical cells with a certain force. This induces a pressure sufficient in certain cases to raise the water to many feet in the xylem. This pressure is called root pressure.
The production of ATP by the phosphorylation of ADP with the help of light energy is called photosynthetic phosphorylation or photophosphorylation. It does not require oxygen and is of universal occurrence in all photosynthetic organisms. There are two types of photophosphorylation: one called cyclic photophosphorylation and the other called non-cyclic photophosphorylation.
Photosynthesis (photo-light, synthesis – putting together).
Photosynthesis may be defined as the process in which the green plants syntehsise carbohydrates from carbon dioxide and water, using the energy of sunlight. The process as it takes place in the green cells of higher plants ma be represented by the following summary equation.
6Co2 + 6H2O C6H12O6 + 6O2
Experiments in Photosynthesis
Experiment 1. That during the process of photosynthesis, oxygen is liberated can be demonstrated by the following experiment.
About half a dozen pieces of water plant (Hydrilla or Elodea) are taken and the cut stem of a glass funnel. The plants are kept submerged with the funnel covering them, in a tall jar or water to which some soda water is added. Over the stem of the funnel is inverted a test-tube filled with water and the apparatus is exposed to bright light (or kept in the sun). Very soon, small bubbles of gas will be seen escaping from the cut ends of the plant and collecting in the test-tube. (Fig.5.3 a). When sufficient gas has collected, the test-tube may be removed and the gas proved to be oxygen by introducing a red-hot splinter.
Fig 5.3 a Experiment to demonstrate the evolution of
oxygen during photo synthesis
Experiment 2. To prove that carbon dioxide is essential for photosynthesis:
If experiment 1 is performed with water which has been previously boiled and cooled, so as to drive away all dissolved gases there is no bubbling from the cut ends. There is no carbon dioxide available for the plant and hence there is no photosynthesis.
Experiment 3. To prove that light is essential for photo synthesis:
The apparatus is set up in experiment 1. But, instead of exposing it to light, it is kept in a dark room. No bubbling of gas from the cut ends is seen. This shows that light is essential for photosynthesis.
Experiment 4. Light screen experiment:
A plant which has been kept in darkness for twenty-four hours in order to make the leaves starch free, is taken. One of the leaves of the plant is tested for starch. There will be no blue colour. To another leaf is attached a light screen which allows a free circulation of air but allows light to pass only through a star-shaped portion of the leaf. The leaf is now exposed to sunlight and tasted for starch after about three hours. It will be seen that only the portion of the leaf that was exposed to sunlight becomes blue, while the unexposed portions are not affected. (Fig. 5.3b).
Fig 5.3 b The light screen experiment
Instead of a light screen, black paper or tinfoil, with stenciled letters or designs, can also be used for the experiment.
This experiment also shows that light energy is not capable of being transmitted from an illuminated to a non-illuminated portion of the leaf. For, if it were capable of being transmitted, starch would have been formed uniformly throughout the leaf.
So, it is clear that in order to perform the function of photosynthesis efficiently, the leaves must be thin and broad. The broader the leaf, the greater is the number of stomata, and so, the greater is the amount of carbon dioxide that can enter the leaf. The thinner the leaf, the less is the solar energy expended.
Experiment 5. To demonstrate the production of starch in green leaves:
A leaf from a bean plant exposed to sunlight is detached in the evening and is dipped in boiling water in order to kill the tissues. Then the leaf is immersed in warm alcohol for some time. The alcohol will dissolve the chlorophyll, and the leaf will get bleached. It will now have a pave a pale white colour. The bleached leaf is dipped in a weak solution of iodine. It will turn blue in colour.
This is due to the presence of starch which has been formed inside the leaf during exposure to sunlight.
Experiment 6. To demonstrate that starch is formed only in those parts of the having the green colour:
For this experiment, a small leaf of Colocasia or Codiaeum which, instead of being uniformly green, shows patches of red or white colour, is selected. Such a leaf is known as a variegated leaf, and it has chlorophyll only in the green portions. The patches are carefully marked on the leaf and then leaf is treated and tasted for starch as in experiment 4.
It will be seen, that the blue colour with iodine is seen only in those portions which were originally green. The other portions show no blue colour because no starch has been formed at these places.
For proper photosynthesis, it is necessary that the entire leaf should be uniformly exposed to sunlight. If only portion of the leaf is exposed, starch will be formed only in that portion.
Experiment 7. Mohl’s Experiment. To shoe that only those portions of the leaf that have direct access to carbon dioxide are capable of effecting photosynthesis:
A starch-free, thin, long leaf detached from a plant kept in darkness for 24 hours is selected for this experiment. A wide-mouthed bottle, provided with a rubber cork split into two halves is taken, and some solution of caustic potash introduced into it. The bottle is kept horizontally on a tray and the leaf fixed in the bottle in such a way that the upper half of the leaf is inside the bottle while the lower half is outside. The petiole of the leaf is kept dipping in a small vessel of water. The apparatus is leaf exposed to the sun. In the evening, the leaf is removed and tasted for starch. It will be found that starch is formed only in the lower half of the leaf. The carbon dioxide in the bottle is absorbed by the caustic potash and so the upper half of the leaf, which was inside the bottle, had no supply of carbon dioxide. And so, there is no starch formed in that half. (Fig. 5.3 c).
Fig 5.3c Mohl’s experiment
5.3.1 Light reaction:
The production of carbohydrate from carbon di-oxide which light is necessary is called the light reaction it is also called hill reaction.
The production of ATP by the phosphorylation of ADP with the help of light energy is called photosynthetic phosphorylation or photophosphorylation. It does not require oxygen and is of universal occurrence in all photosynthetic organisms. There are two types of photophosphorylation: one called cyclic photophosphorylation and the other called non-cyclic photophosphorylation.
Light is a form of electromagnetic energy. Light energy is contained in tiny indivisible units called photons. A beam of light contains a stream of photons. Each photon contains a packet of a definite amount of energy called a quantum. Both the photon and its quantum content are indivisible, The energy of one quantum of red light (average wavelength 700 m?) is about 40,000 calories and of blue light (440 m?) about 64,000 calories. When light impinges upon chlorophyll, the chlorophyll molecules absorb photons and as a result become excited or activated molecules, that is, molecules with more energy than the ground state energy. The extra energy (quanta) absorbed by a chlorcphyll molecule is passed on to one of its electrons which is consequently raised to a higher energy level. The high-energy electron is then expelled from the chlorophyll molecule. Carrying with it the excess energy. As a result of los of electron the chlorophyll molecule assumes a positive charge.
Ch1* (Ch1) +e–
In the above expression hv = light quantum; Ch1* = excited chlorophyll molecule; (Ch1) + = positively charged chlorophyll molecule and e = an electron. The excited electron is than taken up by an electron acceptor, such as a molecule of vitamin K or ferredoxin (an iron-containing protein). From here the electron passes through a chain of cytochromes-iron-containing proteinaceous pigment of chloroplasts-and finally back to the chlorophyll molecule. The union of the electron to the chlorophyll molecule neutralizes its positive charge and the chlorophyll molecule now returns to its normal ground condition, ready to absorb another photon (Fig 5.3 1a). During this journey the electron loses its excess energy which is stored in the energy –rich ATP molecules. The energy of the light is thus converted into the chemical energy of the ATP molecule. Because in this scheme the electron travels in a cycle and ultimately returns to same chlorophyll molecule from which it came. This reaction is called cyclic photophosphorylation. The scheme is illustrated in figure 13.1B.
(ii) Non-Cyclic photophosphorylation. The mechanism of phosphorylation as described above is a primary reaction of photosynthesis. In some photosynthetic bacteria like Chromatium, it is the only photosynthetic reaction. Inmost photosynthetic organisms (green plants and many bacteria), however, illumination is known of the carbon dioxide in the dark. When illuminated, chloroplasts of higher plants are fed with NADP and chloride, omitting vitamin K, the result is the production of NADPH2 accompanied by the evolution of oxygen gas and formation of a small amount of ATP. Arnon suggested another mechanism to explain this evolution of oxygen and production NADPH2. According to this scheme two high-energy electrons expelled from the light-excited chlorophyll molecules are accepted by NADPD and reduce IT TO NADPH2. The protons needed for this reduction of NADP come from the photolysis of water into hydrogen ions (H) + and hydroxyl ions (OH)-. The hydroxyl ions react with one another to produce water and molecular oxygen which is released, as given below:
4H2O 4 H+ + 4 (OH)–
4(OH)– 2H2O + O2 + 4e–
Finally: 2H2O 4 H+ + 4e– + O2
In this process each hydroxyl ion also releases an electron which passes through a chain of cytochrome pigments of the chloroplast. The last cytochrome in the chain in turn donates this electron to the chlorophyll molecule, which earlier had lost an electron, and brings it back to the normal ground state. The energy released during this transfer of electron from the cytochrome molecule is utilized in the formation of ATP by the phosphorylation of ADP.
It will be noticed that in this scheme the electron which was expelled from the chlorophyll molecule does not return to the chlorophyll molecule as happens in cyclic phosphorylation but is used in the reduction of NADP to NADPH2 and the electron that returns to the chlorophyll molecule is derived from an outside source which in this case is the hydroxyl ion of water. Hence this mechanism is termed non cyclic photophosphorylation. In this process oxygen gas is released and both NADPH2 and ATP are produced as illustrated in figure 13.2 Chloride ion (C1-) is essential for the release of oxygen. The net result is the formation of NADPH2 and ATP and the liberation of oxygen.
Osmosis can be defined as a special case of diffusion in which only solvent but not solute molecules diffuse when a solution is separated from its own solvent or a dilute solution is separated from a concentrated solution by a semipermeable membrane.
5.3.2 The Calvin Cycle (C3 Pathway)
Photosynthesis is concerned with the reduction of Co2 which light is not essential. Hence it is called the dark phase of photosynthesis.
Various steps of the Calvin-cycle which has also been shown in are given below:
i) At first the Co is accepted by a five carbon compound ribulose 1,
5-diphosphate already present in the cell and a 6-carbon compound is formed it is unstable. It soon breaks into 2 molecules of three carbon compound called as 3-phosphoglyceric 3-PGA) ACID. Both these reactions take place in the presence of carboxydimutase. 3-phosphoglyceric acid is the first stable product of dark reaction of photosynthesis
Ribulose 1, 5-diphosphate
+Co2 6-carbon 3PGA (2 molecules)
ii) 3- phosphoglyceric acid is reduced to 3- phosphoglyceraldehyde by the assimilatory power (generated in light reaction) in the presence of triose phosphate dehydrogenase. This reaction takes place in two steps:
3 PGA 1, 3 Diosphoglyceric Acid
1, 3 Diosphoglyceric Acid 3phosphoglyceral dehyde
iii) Some of the molecules of 3 – phosphoglyceraldehyde is converted into dihydroxyacetone phosphate, both of which then unite in the presence of the enzyme aldolase to form fructose 1,6-diphosphate:
3- phosphoglyceraldehyde Dihydroxyacetone
3- phosphoglyceraldehyde + Dihydroxyacetore phosphate
Fructose 1, 6-Diphosphate
iv) Fructose 1, 6-diphosphate is converted into Fructose 6-phosphate in the presence of phospharase.
Fructose 1, 6 Diphosphate Fructose–6 phosphate
v) (v) Fructose 6-phosphate (hexose sugar) can easily be converted into a number of other carbohydrates such as glucose, sucrose starch etc.
vi) Some of the molecules of 3- phosphoglyceral dehyde produces in step (ii) instead of forming hexose sugars, are diverted to regenerate ribulose 1,5 diphosphate in the system as follow:
vii) 3- phosphoglyceraldehyde reacts with fructose 6-phosphate in the presence of the enzyme transketolase to form erythrose 4-phosphate (4C, atoms sugar) and xylulose 5-phosphate (5-C atoms sugar)
viii) Erythrose 4-phosphate combines with dihydroxyacetone phosphate in the presence of the enzyme aldolase to form Sedoheptulose 1,7 diphosphate (7-C atoms sugar).
ix) Sedoheptulose 1, 7 disphosphate loses one phosphate group in the presence of phosphatase to form sedoheptulose 7 phospate.
x) Sedoheptulose 7- phosphate reacts with 3- phosphoglyceraldehyde in the presence of transkelolase to form xylulose 5-phosphate and ribose 5 phosphate (both 5-carbon atoms sugars).
xi) Xylulose 5-phosphate is converted into another 5C, atoms sugar ribulose 5-phosphate is finally converted into ribulose 1, 5 diphosphate in the presence of phorphopento kinase and ATP, thus completing the Calvin cycle.
Since the first visible product of this cycle is 3-phosphoglyceric acid which is a 3C compound, calvin Cycle is also known as C3 pathway.
Fig 5.3.2 a Schematic representation of calvin cycle
The metabolic proess in plants in which organic substances are broken down to simpler products with the release of energy is called respiration.
Experiments in Respiration
Experiment 1. To demonstrate the evolution of CO2:
Some fresh flowers or soaked seeds are kept in a bottle. They respire actively and the oxygen in the bottle is utilized and carbon dioxide liberated. The air in the bottle is displaced by adding water through a thistle funnel and the displaced gas is led into a tube of lime-water which will turn milky.
(Fig. 5.4 a).
Fig 5.4 a Experiment to demonstrate respiration
Experiment 2. To show that CO2 evolved is equal to O2 absorbed:
Respiration can also be demonstrated by using Ganong’s respiroscope. This piece of apparatus is somewhat like a report but with no opening for a stop cock. Respiratory material—fresh flower buds or soaked seeds—is introduced into the bulb of the respiroscope, and the narrow end is dipped in a beaker of water. Another respiroscope is set up in a similar manner but with the narrow end dipping into a solution of caustic potash. After some time it will be seen that while the caustic potash solution has risen up in the respiroscope, there is no change in the level of water in the first apparatus. (Fig.5.4 b)
Fig 5.4 b Ganong’s respiroscope
In the second apparatus the carbon dioxide released during respiration has been absorbed by the caustic potash solution and hence there is a rise. Since the volume of oxygen taken in is equal to the volume of carbon dioxide liberated, there is no change in the level of water in the first respiroscope. This is also evident from the equation cited already.
Evolution of heat during respiration. All the energy that is released during respiration is not made of by the plant. A small amount of the energy is dissipated in the from of heat, so that there is a rise in temperature. In most plants it is not perceptible but can easily be observed in fresh flowers and germinating seeds.
Experiment 3: To show evolution of heat during respiration:
Some fresh flowers or soaked seeds are kept in a vacuum flash. A thermometer is introduced into the flask through a hole in the cork, and the bulb of the thermometer is kept dipping in the respiratory material. The material respires actively, heat is produced and the temperature rises.
Fig 5.4 c Evolution of Heat during respiration
Glucose is finnaly broken down to pyruvic acid or pyruvates. This process is called glycolysis. The conversion of a hexose sugar to water and CO2 takes place through a series of integrated reactions. Enzymes responsible for these changes are phosphorylases or transphosphorylases, dehydrogenases, etc.
First occurs phosphorylation of sugar. This results in the formation of glucose-6-phosphate. The enzyme involved is hexokinase.
Glucose + ATP + Hexokinase ? Glucose-6- phosphate + ADP.
The glucose-6-phosphate is then isomerized to fructose-6-phosphate by the action of an enzyme, phosphohexoisomerase.
The next step is the formation of fructose-1, 6-disphosphate which is effected through the participation of ATP and an enzyme, called phosphofructokinase.
Fructose-6-phosphate + ATP + Phosphofructokinase ? Fructose-1,
6- diphosphate + ADP.
Now occurs a breakdown of six-carbon compounds into three-carbon compounds, a process facilitated by an enzyme, called Aldolase. As a result, 3-phosphoglyceraldehyde (triosephosphate) and dihydroxyacetone phosphate are formed.
Fructose-1, 6-diphosphate 3-phosphoglyceraldehyde.
This marks the end of the first stage of glycolysis.
The conversion of phosphogyceraldehyde to pyruvic and is the second stage of glycolysis. During the first stage, we dealt with phosphate donors. But here, we are concerned with hydrogen donors and oxidation-reduction changes in the nature of hydrogenation and dehydrogenation are effected through the agency of called hydrogenases. The hydrogen donars in question are diphosphopyridine mucleotide (DPN) and triphopyridine mucleased (TPN).* In this group of reactions, the ADP is converted to ATP by means of the phosphate group that happens to be released in the course of the formation of pyruvic acid. The 3-phosphoglyceraldehyde, which is the end product of glycolysis, stage I, is converted into 1, 3-diphosphoglyceric acid through the intervention of DPN (co-enzyme A) and triosephate dehydrogenase. One of the phosphate of this 1, 3-diphosphoglyceric acid is taken over by ADP to become ATP and itself gets converted into is 3-phosphoglyceric acid through agency of the enzyme, triosephosphate transphosphorylase. An enzyme called phosphogyceric mutase now converts 3-phosphoglyceris acid into 2- phosphoglyceric acid, which by a process of condensation effected through the agency of enolase, results in the formation of 2-phosphopyruvice acid. The phosphte to group is now broken loose and employed in the conversion of ADP into ATP and with this pyruvic acid is formed. With the formation of pyruvic acid, glycolysis is complete.
We may now summarize gycolysis as follows:-
(1) Glusoce + ATP Glucose-6-phosphate + ADP.
(2) Glucose-6- phosphate Fructose-6-
(3) Fructose-1,6-disphosphate + ATP Fructonse-1,
6-diphosphate + ADP.
(4) Fructose-1, 6-diphosphate Dihydroxyacetone phosphate + 3
(5) 3-phosphoglyceraldehyde + DPN 1, 3-
acid + DPN.H2.o
(6) 1, 3-diphosphoglyceric acid + ADP 3-
phosphogyceric acid + ATP.
(7) 3- phosphoglyceric acid 2-phosphoglycric
(8) 2-phosphoglyceric acid 2-phosphopyruvic acid.
(9) 2-phosphopyruvic acid + ADP Pyruvic
acid + ATP.
The formation of pyruvic acid marks the end of the common path of the E.M.P. pathway (the Embden-Mayer-Parnas pathway). Pyruvic acid which is the end-product of the glycolytic pathway, may be formed by other alternate pathway also: it may be formed from some of the intermediate products of glycolysis or it may be formed from amion acids or during the oxidation of fats. It is the fate of pyruvic acid that is of greater significance rather than how it came to be because the fate of this compound, through the sequence of reactions of the aerobic phase of respiration, yields to the organism a greater portion of energy obtained by the process of respiration. At this point, the presence or absence of oxygen determines the direction of the further break-up of pyruvic acid. In the absence of oxygen it is broken into ethyl alcohol, with acetaldehyde being formed as an intermediate substance. On the other hand, in the presence of oxygen, the pyruvic acid which contains hand, in the presence of oxygen, the directly toGO2. In the course of this degradation the reactions proceed in the from of a cycle, variously called tricarboxylic acid cycle, citric acid cycle or Krebs cycle.
5.4.2 Krebs Cycle
The concept pf Krebs cycle as a concept of pyruvate- oxidation is based upon the work of Krebs and Jhnson in 1937. Through their work is done mainly in relation to the muscular tissue of the animals, the following facts show that the cycle is recognizable in plants also: (1) the various substance figuring in the TCA cycle are present in the plant cells also; (2) the various enzymes that came to be recognized as operative in the cycle are recognized in plant cells; and (3) the capacity of the plant tissue to metabolize the various acids in question is proved beyond doubt.
The essential outline of the Krebs cycle is as follows: First, the pyruvic acid is decarboxylated, i.e., the CO2 is split out of the carboxyl group. It should be remembered that pyruvic acid has three carbons in it. As a result of this loss of one carbon, a 2-carbon fragment remains and this gets bound to co-enzyme A or Co-A, forming Acetyl-Co-A. The Acetyl-Co-A is than coupled to the 4-carbon acid known as oxalo-acid, forming a 6-carbon tricarboxylic acid, the citric acid, releasing Co-A in the process.
The formation of citric acid may be said to initiate the Krebs cycle or the citric acid cycle. In the ensuing reactions, the citric acid is rearranged, by addition and removal of water, to form isocitric acid. The isocitric acid is then oxidized to from a keto acid that loses CO2 forming ?-Ketoglutaric acid. Oxidative decarboxylation of the ?-Ketoglutaric results in the formation of succinic acid and CO2. The succinic acid is then oxidized to furmaric acid, which upon hydration leads to the formation of malic acid. The oxidation of malic acid regenerates the oxaloacetic acid, and thus the cycle reches its starting point once again. If all the enzymes necessary are present, this cycle can be initiate by the addition of any of the compounds within the cycle, through a supply of Co-A is to be maintained.
In the process of these conversions, hydrogen is release and accepted by a number of hydrogen acceptors present in the cell. The DPN, Flavoprotein, and the cytochromes are among the chief hydrogen acceptors of the cell. Later, the hydrogen acceptors come under the influence of oxidases and in this process of dehydrogenation, ADP, gets converted into ATP, a substance that contains a high-energy phosphate group. In fact, the whole purpose of the respiratory process, as it were, lies in the formation of as may ATP molecules as possible and from this point of view, aerobic decomposition of glucose to CO2 and water is far more efficient than the anaerobic decomposition of that substance to pyruvic acid, because the former process yields 36 more molecules of ATP than the latter.
The reactions of Krebs cycle may be summed up as follows:
(1) Pyruvix acid + Coenzyme A + NAD ? Acety1 coenzyme A + NADH -Co2 + H+.
In this reaction decarboxylation (removal of CO2) and dehydrogenation occur.
(2) Acetyl-Co-A + Oxaloacetate ? Citric acid + Co-A.
The co-enzyme a produced in this reaction is again available for further breakdown of pyruvic acid.
(3) Citric acid Isocitric acid.
(4) Isocitric acid + NADP Oxalosuccinic acid+NADPH+ H+
While all the other reactions of Krebs cycle are NAD dependent, this one reactions is NADP specific.
(5) Oxalosuccnin acid ?-Ketogluataric acid + CO2
(6) ?-Ketogluataric acid is converted to succnin acid through two steps:
(a) ?-Ketogluataric acid + NAD + Co-A Succinyl Co-A + NADH +Co2.
(b) Succinyl Co-A + H2 O ® Succinic acid + Co-A.
Reaction 6 is strongly exergonic and ADP, if available at the site, is converted to ATP, regenerating Co-A and succinic acid
(7) Succinic acid suffers dehydrogenation, resulting in the formation of fumaric acid.
Succinic acid + NAD Fumaric acid + NADH + H+.
(8) Fumaric acid + H2O Malic acid.
(9) Malic acid + NAD Oxaloacetate + NADH + H+
Fig 5.4.2 Schematic representation of Kreb’s cycle
The oxaloacetate is utilized in the formation of citric acid in combination with Co-A or is converted to aspartic acid by a process of amination. If oxaloacetic acid gets accumulated, then the progress of the cycle will be arrested.
As can be seen from a perusal of the steps making up Krebs cycle, apart from the specific enzymes facilitating individual reactions, the Co-A, NAD (DPN) and NADP (TPN) play important roles in the maintenance of the cycle.
The Krebs cycle is significant in more ways than outlined above. First of all, it can be initiated by oxidation of pyruvic acid, derived not necessarily by the anaerobic decomposition of glucose but even from fat metabolism and amino acid metabolism. This fact makes Krebs cycle not dependent upon carbohydrate metabolism only.
Krebs cycle also provides fragments and reactions for synthesis, as many of the reactions characterizing the cycle are reversible. For example, when the cells of an organism need a particular amino acid, say glutamic acid, the a-Ketogluataric acid present as one of the tricarboxylic acids in the cycle can be readily aminated to from glutamic acid. Similarly, acetic acid can be aminated to from glycine. If fats are required, the 2-carbon acetyl molecules and the 4-carbon acids can be linked to bring about the formation of fatty acids, out of which fats may be built.
The Krebs cycle thus occupies a key position in what has come to be called metabolic mill.
They grow on extremely wet soil where water is available to plants in abundance. According to the way in which they develop in water, they are further subdivided into the following five categories:
I) Free- floating hydrophytes
They remain in contact with water and air, but not soil. They float freely on the water surface. Leaves in some are very minute, while in others quite large. Some of the free-floating hydrophytes, as Wolffia, Lemna, Spirodella, Azolla, Eichhornia, Salvinia and Pistia, are shown in Figure 5.5a.
Fig 5.5a Free floating hydrophytes
II) Rooted hydrophytes with floating leaves
Their roots are fixed in mud, but leaves have long petioles which keep them floating on the water surface. The remaining plant, except leaves, remains in water. Some of the rooted hydrophytes with floating leaves, as Trapa, Nelumbo, Nymphaea and Marsilea are shown in Figure 5.5 b.
Fig. 5.5 b Rooted hydrophytes with floatin leaves
III. Submerged floating hydrophytes
These are the plants in contact with only water, being completely sub- merged and not rooted in the mud. Their stems are long and leaves generally small some of the examples are Ceratophyllum, Utricularia, Najas, etc. In Ceratophyllum, (Fig. 5.5c), roots are lacking even in embryo stage, sometimes leafy branches being modified into ‘rhizoids’.
Fig 5.5 c Submerged hydrophytes
Fig 5.5 d Rooted submerged hydrophytes
IV. Rooted submerged hydrophytes
These are hydrophytes like Hydrilla, Potamogeton, Isoetes and Vallisneria (Fig.5.5 d) that remain completely submerged in water and rooted in soil. In Hydrilla, Potamogeton and Chara, the stem is long, bearing small leaves at the nodes, Hydrilla is a slender weed with fibrous roots. In Isoetes and Vallisneria stem is tuberous (corm-like) with long leaves, which are narrow, ribbon shaped.
V. Rooted emergent hydrophytes
The grow in shallow waters. These are such hydrophilous forms which, although require excess of water, but their shoots (assimilatory organs) are partly or completely exposed to air. The root system is completely under water, fixed in soil in some, as Sagittaria and Ranunculus (Fig.5.5e), shoots are partly in water and partly emerging i.e. exposed to air, whereas in others such as Scirpus and Cyperus (Fig.5.5e) also known as marsh plants, the shoots are completely exposed to air.
Fig 5.5e Rooted emergent hydrophytes
Due to availability of water in plenty roots, the principal organs of water absorption, in such plants become of secondary importance, and less significant. Their overall development is usually very poor and insignificant in most of the hydrophytes.
i) Roots may be entirely absent as in Wolffia, Salvinia (Fig.5.5a) and Ceratophyllum (Fig.5.5c) or poorly developed as in Hydrilla (Fig.5.5d). In Salvinia (Fig.5.5a) submerged leaves compensate for roots. However, in emergent forms, as Ranunculus (Fig.5.5e) which grow in mud, roots are well developed with distinct root caps.
ii) Root hairs are absent or poorly developed.
iii) Root caps are usually absent. In some cases, as Eichhornia (Fig.5.5a), root caps are replaced by ‘root pockets’.
i) In submerged forms as Hydrilla and Potamogeton (Fig.5.5d) stem is long, slender, spongy and flexible. In free- floating forms it may be slender, floating horizontally on water surface as in Azolla (Fig.5.5a) or thick, short, stoloniferous and spongy as in Eichhornia (Fig.5.5a). In forms which are rooted with floating leaves it is a rhizome, as in Nymphaea and Nelumbo (Fig.5.5b).
i) In submerged forms, leaves are thin, and are either long and ribbon- shaped as in Vallisneria (Fig.5.5d), or long and linear as in Potamogeton (Fig.5.5d), of finely dissected as in Ceratophyllum (Fig.5.5c). Floating leaves are large, flat and entire as in Nymphaea and Nelumbo (Fig. 5.5b) with their upper surfaces coated with wax; their petioles are long, flexible, and often covered with mucilage. In Eichhornia (Fig.5.5a), and Trapa (Fig.5.5b) petioles become swollen and spongy.
4. Flowers and seeds:
These are less common in submerged forms. Where flowers develop, seeds are rarely formed.
The distribution of various tissues in roots of hydrophytes, in general, becomes clear from Figures 5.5f and 5.5g showing transverse section of roots of Potamogeton and Eichhornia respectively. It may be concluded from these figures that:
i) Cuticle is either completely absent or, if present it is thin and poorly developed.
ii) Epidermis is usually single- layered made up of thin- walled parenchymatous cells.
iii) Cortex is well developed, thin –walled and parenchymatous, major portion of which is occupied by well developed prominent air cavities- the ‘aerenchyma’ which offers resistance to bending stress, increases buoyancy and allows a rapid gaseous exchange.
iv) Vascular tissues are poorly developed and least differentiated in sub-merged forms, as Potamogeton (Fig.5.5f) with thin-walled elements. In xylem, vessels are less common, tracheids being generally present. In floating forms as Eichhornia (Fig.5.5g) they are comparatively differentiated to some extent. However, in emergent forms as Ranunculus and Typha etc., vascular elements are comparatively much distinct and well developed.
v) Mechanical tissues are generally absent except in some emergent forms, as Typha where pith cells are sclerenchymatous.
Distribution of various tissues in stems of hydrophytes becomes clear from transverse section of stem of Hydrilla (Fig.5.5h). It may be seen that:
i) Cuticle is either absent or poorly developed and thin.
ii) Epidermis is usually single –layered made up of thin-walled parenchymatous cells. However, rhizomes of Nymphaea and Nelumbo show well- developed epidermis. In emergent forms as Typha, cuticle as well as epidermis are generally well developed.
iii) Hypodermis is completely absent in submerged form as Hydrilla and Potamogeton. However, in floating and emergent forms, it may be present as thin walled parenchyma or collenchyma.
iv) Cortex is well developed, thin-walled and parenchymatous, extensively traversed by air cavities as in roots. Cells of cortex generally possess chloroplasts and are thus photosynthetic. In some, as Nymphaea, there are found large number of vascular bundles scattered in the cortex.
v) Endodermis is generally distinct, especially in rhizomes and similar organs.
vi) Vascular bundles generally lack bundle sheaths. Vascular elements are thin-walled, lignified elements being absent. However, in emergent forms as Typha, vascular elements are comparatively well differentiated and developed.
vii) Mechanical tissues are usually absent.
Fig 5.5h T.S of stem of Hydrilla
Fig 5.5 i T.S of leaf of Anacharis
Fig 5.5 j T.S of leaves Anacharis pusillus
It their internal structure, leaves in different hydrophytes show variations. However, some of the anatomical features are common to most of the leaves. Distribution of different tissues in leaves of hydrophytes would become clear from Figures 5.5 i, 5.5j and Fig 5.5 k showing vertical transverse sections of leaves of Anacharis, Potamogeton and Nymphaea respectively. From these we may conclude that in leaves:
i) Cuticle is usually absent in submerged forms as Anacharis and Potamogeton (Figs.5.5i, 5.5j). In floating forms as Nymphaea (Fig.5.5k) stomata are confined only to upper side, and is thin. In emergent forms also it is thin.
ii) Epidermis is single- layered, made up of thin-walled cells with abundance of chloroplasts.
iii) Stomata are completely absent in submerged leaves as Anacharis and Potamogeton (Figs. 5.5i, 5.5j), In floating forms as Nymphaea
(Fig. 5.5k) stomata are confined only to the upper surface of leaf, whereas in emergent forms they are generally found on both surfaces of leaves.
iv) Mesophyll is undifferentiated in submerged leaves, where it is generally single- layered as in Potamogeton (Fig. 5.5j). In Anacharis (Fig.5.5i) also it is thin and undifferentiated. In floating leaves as Nymphaea (Fig.5.5k), it is, however, differentiated into palisade and spongy parenchyma with well-developed air cavities. In emergent leaves, mesophyll is well differentiated with air cavities.
Fig 5.5 l T.S of Petiole – Nymphaea
v) Vascular tissues are very much reduced and sometimes difficult to be differentiated into xylem and phloem as in Nymphaea (Fig.5.5k), xylem elements are thin- walled, phloem being well developed. However, in aerial leaves, vascular elements are comparatively well differentiated with vessels in xylem.
vi) Mechanical tissues are absent.
vii) The petioles, where well developed, as in Nymphaea (Fig. 5.5l) also possess internally the various tissues characteristic of a typical hydrophyte, i.e an abundance of aerenchyma, thin-walled cells, lack of differentiation in vascular tissues and absence of any lignified mechanical tissues.
Fig 5.5 k T.S leaf of Nymphaea
The term ‘xerophyte has been defined and interpreted variously. Morphological features and rates of transpiration have almost failed to explain the true nature of this group of plants. They are sometimes loosely defined as ‘plants of dry habitats’. But if taken in this sense, most of the mesophytes should be called xerophytes. Daubenmire (1959) in his discussion on the group defined xerophytes as ‘plants which grow on substrata that usually become depleted of growth water to a depth of at least 2 decimeters during a normal season’.
On the basis of their morphology, physiology and life cycle pattern, xerophytes are generally classified into the following three categories:
I. Ephemeral annuals
They are also called as ‘drought evaders’ or ‘drought escapers’. They are mostly found in arid zones. They are annuals, which complete their life cycles within a very short period of 6-8 weeks or so. With their small size and large shoots in relation roots, they are well adapted to such dry habitats. They actually avoid and not withstand dry seasons, and thus escape dryness in external and internal environments. Some do not prefer to call them true xerophytes. Examples- Argemone mexicana, Solanum xanthocarpum, Cassia tora, Tephrosia purpurea, etc.
They are the plants that suffer from dryness in external environment only. Their succulent, fleshy organs (stems, leaves, roots) serve as water- storage organs which accumulate large amount of water during brief rainy season. In cacti moreover, the root systems also become shallow. Their root system is shallow, stem swollen and leaves thick, leathery and succulent. Some of the examples are Aloe, Euphorbia and Opuntia (fig.5.5m) and various cacti, Agave and Ceiba parviflora. As the succulents avoid drought, some prefer to exclude them from true xerophytes. This is indeed a unique mode of adaptation.
Fig 5.5m Succulents xerophytes
III. Non – succulent perennials
These are actually the true xerophytes or drought- resistants, because they possess a number of morphological, anatomical and physiological characteristics which enable them to withstand critical dry conditions. They are the plants that suffer from dryness both in their internal as well as external environments.
Non- succulent perennials like Calotropis procera, Acacia nelotica, Zizyphus jujuba and Cappa is aphylla are shown in Figure 5.5n. Other examples are Prosopis, Casuarina, Nerium, Alhagi, Saccharum, and Salvadora.
Fig.5.5n Non-succulent perennials.
The general characteristics features of non-succulent xerophytes are as follows:
1. Root system is very extensive. For example, in Prosopis, Calotropis and Alfalfa, roots may be more than 30 meters long.
2. Following characteristics helps minimizing the rate of transpiration:
i) dying back of leaves as in many grasses;
ii) rolling and folding of leaves as in many grasses like Ammophila arenaria;
iii) delicate leaves which are shed under conditions of less supply of water;
iv) heavy cuticular and epidermal layers;
v) waxy coatings on leaves;
vi) sunken type of stomata on leaves;
vii) very small, narrow, sometimes scaly leaves;
viii) ridged stems.
3. Aerial organs may become variously modified according to the prevailing climatic conditions.
4. Experimentally, the reduction of water supply has been shown to reduce the shoot/ root ratio in some grasses.
In contrast wit hydrophytes which develop in conditions with plenty of water, xerophytes develop under water- deficient conditions. Thus, in order to secure water, which is present in less amount and, moreover, in deeper levers of soil, roots in xerophytes become the principal organs of primary importance. The root system is thus very well developed, with the following characteristics:
i) It is very extensive, which in some cases is several time slonger than the shoot. Roots are long, tap roots, with extensive branching spread over wide areas.
ii) Root hairs and root caps are very well developed.
i) Mostly they are stunted, wood, dry, hard, ridged and covered with thick bark.
ii) In some as Saccharum, stem become sunderground, whereas in Opuntia (Fig. 5.5m) it becomes fleshy, green, leaf-like (phylloclade) covered with spines. In Euphorbia also (Fig.5.5m) it becomes fleshy and green.
iii) On stems and leaves there are generally hairs and / or waxy coatings.
i) Leaves are very much reduced, scale- like appearing only for a brief period, sometimes modified into spines. Lamina may be long, narrow or needle- like as in Pinus or divided into many leaflets as in Acacia.
ii) Foliage leaves, wherever present, may become thick, fleshy and succulent, or tough and leathery in texture.
iii) Leaf surfaces are generally shiny and glazed to reflect light and heat.
Fig 5.5p T.S leaf of Peperomia
iv) In some monocots as Ammophila, Poa and Agropyron, leaves become folded and rolled in such a manner that the sunken stomata become hidden, and thus rate of transpiration is considerably minimized.
v) In some of them as Euphorbia (Fig. 5.5m), Acacia nelotica, Zizyphus jujube and Capparis aphylla (fig. 5.5n), stipules become modified into spines.
i) Roots hairs and root caps are well developed. In Opuntia, root hairs develop even at the root tips,
ii) Roots may become fleshy to store water as in Asparagus.
iii) In Pinus edulis and Calotropis, roots possess rigid and thickened walls.
i) In succulent xerophytes, stems possess a water- storage region.
ii) In stems of most of the non-succulent xerophytes, such as Casuarina (Fig.5.5 o), there are present the following chief characteristics:
Fig 5.5 o T.S stem of Casuarina
a. Cuticle is very thick
b. Epidermis is well developed, with heavily thickened cell walls.
c. Hypodermis is several- layered and sclerenchymatous.
d. Stomata are of sunken type.
e. Vascular tissues are very well developed, differentiated, heavily lignified.
Vascular bundles have well developed several- layered bundle sheaths.
f. Mechanical tissues are very well developed.
iii) Barks is very well developed.
iv) Oil and resins are often present
i) In succulent leaves of malacophyllous xerophytes, such as Peperomia (Fig.5.5p), epidermal cells of leaves serve as water- storage organs.
Similarly, succulent leaves of Aloe (Fig.5.5q), and Salsola have prominent water –storage regions in their mesophyll. Moreover, in such leaves cuticle is thick and outer walls of the epidermal cells are heavily deposited with cutin and cellulose.
ii) Leaves of non-succulent xerophytes, such as Nerium (Fig. 5.5r) and Pinus (Fig 5.5s) etc., possess:
Fig 5.5q T.S leaf of Aloe
a) Well developed heavy cuticle.
b) Several- layered epidermis in Nerium, and several- layered, sclerenchymatous hypodermis in Pinus.
c) Mesophyll very well differentiated into palisade and spongy parenchyma.
d) Stomata of sunken type confined only to lower epidermis. In some xerophytes as Nerium (Fig.5.5r), stomata are situated in pits lined with hairs.
e) Vascular tissues very well developed, differentiated into xylem with lignified elements, and phloem. In Nerium, in addition to big vascular bundle in mid-rib region, there are several other vascular bundles also.
f) Mechanical tissues very well developed, including several kinds of sclereids. In Pinus, there is well developed complex transfusion tissue.