Leaf is an important bifacial (two surfaced) photosynthetic organ of a plant. Anatomy is the study of the internal structure of an organ. Internally the leaf consists of various tissues. These tissues are arranged in a definite pattern internally in the leaf. The various tissues present inside the leaf will carry out various functions. To know the physiological activity of the leaf we must have a clear understanding of the internal structure of the leaf. By taking a thin section of any Dicot leaf, mounting on a clean glass slide after staining and observing it under a normal Compound microscope, we can have a clear picture of the internal structure of a Dicot leaf.
Structure of a Dicot leaf as observed under a Microscope
A Transverse Section (T.S.) of a Dicot leaf shows the following internal organization.
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1. Upper epidermis
It is the uppermost layer or adaxial layer of a dicot leaf. It is a single layered tissue made of cubical cells and arranged closely with no inter cellular spaces in between them. Chloroplasts cannot be seen in these cells. Upper epidermis on its outer surface is covered by a thin cuticle. The cuticle present on the upper epidermis is comparatively more thicker than the lower epidermis. Thick cuticle is mainly meant to check the loss of water through the surface of the leaf. On the upper epidermis less number of stomata are present when compared to the lower epidermis in a dicot leaf. Presence of less number of stomata on the upper surface is an adaptation to reduce the loss of water through transpiration.
Mesophyll is a ground tissue present in between upper epidermis and lower epidermis. It consists of two tissues- Palisade parenchyma and Spongy parenchyma. Palisade parenchyma is present just below the upper epidermis. This tissue is present in one or two layers. The cells of palisade parenchyma are very much elongated and columnar in nature. The cells are closely packed in each layer with narrow inter cellular spaces in between them. The cells of this layer internally include more number of chloroplasts in them. As chloroplasts are more in number in these layer, chlorophyll pigment present also will be more. Due this only, the upper surface of a dicot leaf will be more green in nature when compared to the lower surface.
In between the palisade parenmchyma and lower epidermis, spongy parenchyma is present. The cells of spongy parenchyma are small in size and irregular in shape. The cells of spongy parenchyma are loosely arranged with lot of large intercellular spaces in between them. The cells of spongy parenchyma are with less number of chloroplasts and less quantity of chlorophyll. Due to this only, the lower surface of a dicot leaf will be less green in color when compared to upper surface. As the mesophyll tissue i.e. both palisade parenchyma and spongy parenchyma include chloroplast as well as chlorophyll concerned with food preparing process photosynthesis.
3. Vascular Bundles
Vascular bundles are present within the mesophyll tissue. They represent the midrib and veins of a leaf. Each vascular bundle consists of xylem and phloem complex tissues surrounded by bundle sheath. Xylem tissue of a vascular bundle is present towards the upper epidermis of the leaf. Xylem complex permanent tissue consists of xylem tracheids, xylem vessels, xylem parenchyma and xylem fibers. Xylem in a vascular bundle is concerned with transport of water and minerals.
Phloem complex tissue is present towards the lower epidermal surface of the leaf. Phloem complex permanent tissue is made of sieve tubes and sieve pores, companion cells, phloem parenchyma and phloem fibers. Phloem tissue in a leaf is concerned with transport of food materials.
In the vascular bundle both xylem and phloem are covered on its outer surface by bundle sheath layer.The bundle sheath layer is made of single layered thin walled parenchymatous cells. The cells in the bundle sheath contain chloroplasts and hence this layer is also concerned with photosynthesis. In old leaves sclerenchymatous extension can be seen from this layer towards upper and lower epidermis. Vascular bundles in dicot leaf are described as conjoint, collateral and closed. As xylem and phloem are present on the same radius, the vascular bundle is described as conjoint and collateral. The vascular bundle is described as closed as there is no cambium present between xylem and phloem. Vascular bundles help in transport of water, minerals and food in the leaf. Vascular bundles also provide strength to the leaf.
4. Lower epidermis
Below the spongy parenchyma a single layer of epidermis is present. This layer is present on the abaxial surface of the leaf. The cells are cubical in shape and are arranged very closely without any inter cellular spaces. Many stomata are present on the lower surface of the leaf in comparison to the upper surface of the leaf. These stomata open during time which allow exchange of gases through diffusion and the escape of water vapor. Just above the stomata on the lower surface of the epidermis air cavity or sub-stomatal chamber is present. This air cavity acts as a store house of carbon dioxide or water vapor till they diffuse.