The dicot leaves have two side the upper side and the lower side. The upper side is directed above and the lower side is directed below so these leaves are known as bifacial leaf or dorsiventral leaves. The leaves obtain more sunlight on the upper side than on the lower side so there is difference in the anatomical structure and the tissue orientation on either side. The internal structure may be differentiated into;
(i) Epidermis: The epidermis is present on both sides so known as the upper epidermis and lower epidermis. The epidermis is made up of uniseriate cells, which are compactly arranged, thin walled and parenchymatous. On the upper epidermis there is the presence of thick cuticle layer but it is thinner on the lower surface. The cuticle prevents the greater evaporation of water from the surface of the leaves. The stomata are present on the lower side of the epidermis so the known as the hypostomatal leaf.
(ii) Mesophyll tissues: The mesophyll tissue is differentiated into two; the palisade tissue and the spongy cells. The palisade tissues are found below the upper epidermis and are arranged at right angles to its cells. These cells are columnar in shape arranged without the inter cellular space. All these cells contain large number of chloroplast arranged along the radial walls of the cells.
The main function of palisade tissue is photosynthesis.
The spongy cells lie below the lower epidermis and these cells are polygonal in shape. These cells are loosely arranged with large inter cellular space. The spongy tissue has greater air spaces, the internal exposed cell surface facilitate gaseous exchange. The water vapour exchange between the cells and inter cellular space this inter ventilating system open through the stomata on the lower epidermis. Number of chloroplast present in the mesophyll cells lesser than the palisade tissue so the lower surface of the leaf appears as pale green.
(iii) Vascular bundles: The vascular bundles are embedded in mesophyll tissue. The bundles are conjoint, collateral and closed. Vascular bundle of the mid vein is large in size than the other bundle. Colorless parenchymatous cell surrounds the bundle and spread up to upper and lower epidermis. These cells are collectively called as the bundle sheath extensions.
The vascular bundles have xylem lying towards the upper epidermis and phloem lies towards the lower epidermis. The xylem is composed of annular and spirally thickened vessels, tracheids, fibres and xylem parenchyma. The proto xylem of the vascular bundle lies towards upper surface and the meta xylem lies towards the lower side. Phloem consists of sieve tubes, companion cells and phloem parenchyma.