IntroductionAnatomy of stem includes the internal arrangement of various tissues in a specific way. Epidermis, hypodermis, ground tissues (Parenchyma), vascular tissues (Xylem and Phloem) are tissues we can find in the anatomical structure of a typical monocot stem. To study the anatomical structure, a thin T.S. of a Maize stem (Zea mays) or Grass stem (Cynodon dactylon) is taken and observed under the low power of a Compound microscope. The following description reveals the specific arrangement of various tissues in general of any monocot stem.
Anatomical structure of typical monocot stemA T.S. of a young dicot stem of Maize stem (Zea mays) shows the following arrangement of tissues from the periphery towards the centre.
The anatomical structure of Monocotyledons Maize stem (Diagram drawn by Author)
EpidermisIt is the outermost single-layered structure consisting of somewhat flattened living cells around the stem. The cells are closely packed in this layer without any intercellular spaces between them. The cells lack any hairs or projections over them but the outer surfaces of the cells are covered with a somewhat thick cuticle. The cuticle layer present on the outer surface of the epidermal layer will prevent the excessive loss of water from inside the plant body. We can observe a few stomata here and there over the epidermal layer. These stomata on the surface of the stem helps in the exchange of respiratory gases.
HypodermisBelow epidermis, 3-4 layers of sclerenchymatous hypodermis is present. Cells are small, polygonal in shape and compactly packed without any intercellular spaces. This tissue is interrupted by here and there by chlorenchyma cells. Hypodermis provide mechanical strength to monocot stem.
Ground tissueThere is no distinction of ground tissue into the cortex, endodermis, pericycle and pith in monocot stem. Inner to the hypodermis, the entire mass of living parenchymatous cells makes up the ground tissue. A large number of vascular bundles covered with dead sclerenchymatous fibre get dispersed in an irregular manner within the ground tissue. The parenchyma layers present below hypodermis are made of small and polygonal-shaped cells and are closely packed. The parenchyma cells present towards the centre of the stem are usually big and round in shape and closely packed but with few intercellular spaces between them in the anatomy of monocot stem. Endodermis and pericycle are absent in the anatomy of monocot stem. Cortex and stele cannot be separated in monocot stem because of the absence of endodermis.
Vascular bundlesVascular bundles are numerous and scattered all over the ground tissue. So the stele in monocot stem is usually described as atactostele. The vascular bundles are scattered from below the epidermis to the centre of the stem. The vascular bundles are more in number in the outer exterior region but their number becomes less as we proceed to the center. The vascular bundles are smaller in size towards the epidermal region while their size is larger towards the center. Each vascular bundle is covered by a thick sheath of the sclerenchymatous fiber called 'bundle sheath'. So the vascular bundles in monocot stem are usually described as 'sheathing bundles'.
The vascular bundles in monocot stem are described to be conjoint, collateral, endarch and closed. The word conjoint indicates that both xylem and phloem are present together, collateral, where xylem and phloem are seen on the same radius, closed means there is no cambium present between xylem and phloem. Endarch vascular bundles mean phloem tissue present in the vascular bundle is present towards the periphery while xylem tissue is located towards the center. As cambium is absent in vascular bundles no secondary thickening can be seen in monocot stems.
Xylem elements in the vascular bundles are arranged in the form of a Y-shape. Protoxylem and metaxylem elements are present in the vascular bundle. The first formed (younger) xylem and metaxylem (older) xylem are present in the vascular bundles. The metaxylem elements form the arms of 'Y' and the protoxylem forms the junction of 'Y'. The protoxylem elements dissolved to form a lysigenous cavity called Protoxylem lacuna or protoxylem cavity. Protoxylem elements include few vessels with annular or spiral thickenings. The presence of the 'lysigenous cavity' is a characteristic feature of a monocot stem. Metaxylem elements are composed of two large vessels with pitted thickenings. Metaxylem tracheids are present between two metaxylem vessels.
The phloem of vascular bundles lies outer to xylem elements. The phloem tissue of vascular bundles is composed of sieve tubes and companion cells. The phloem parenchyma element is absent in the phloem tissue of monocot stems. The absence of medullary rays is another identifying characteristic feature of monocot stems.
Identifying anatomical characteristic features of Monocot stem in comparison to Dicot stem
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