Describe the internal structure of a dorsiventral leaf with the help of labelled diagrams.

Explanation

Transverse section of a dorsiventral leaf (dicot leaf ) shows 3 important parts . They are : 1 .Epidermis , 2 Mesophyll and 3. Vascular bundles .
1 . Epidermis : Epidermis is present on the both the upper surface (adaxial ) and the lower surface (abaxial ) of the leaf . The epidermis present on the adaxial surface is called upper epidermis and on the abaxial surface is called lower epidermis . The epidermis is amde up of one row of barrel shaped cells , which are arranged compactly without intercellular spaces. The cells are filled with cacuolated and nucleated protoplast . On outerside of the epidermis a waxy layer called Cuticle is present .Stomata are present , more on the lower surface than on the upper surface . each stoma is surrounded by two kidney shaped guard cells . They are chlorophyllous and regulate the opening , and closing of stomata . Epidermis shows multicellular uniseriate hairs . The cells of leaf hairs are filled with water . They protect the inner tissues by absorbing the uniseriate hairs . The cells of leaf hairs are filled with water . they protect the inner tissues by absorbing the heat and prevents evaporation of water from the leaf surface . The stomata help in the gaseous exchange and also promote transpiration .
(2) . Mesophyll : The ground tissue that extends betweeen the upper and lower epidermal layers is called the mesophyll . It is composed of thin walled parenchyma with chloroplasts . It is chiefly concerned with the synthesis of carbohydrates .In dicot leaves mesophyll is differentiated into two parts namely ,
(i) Palisade parenchyma : Part of the mesophyll found towards the lower epidermis is called spongy tissue . It shows 3-5 rows of irregular shaped cells that are arranged loosely with large intercellualr spaces . Some intercellular spaces present in the vicinity of the stomata are very large , forming air chambers green and the lower surface is light green in colour . Spongy tissue has a primary role in gaseous exchange apart from the synthesis of food materials .
(3) . Vascular bundles Vscular bundles are extended in the mesophyll in the form of veins . They help in supplying water , mineral salts and food materials all over the leaf surface . Veins also provide mechanical strength to the leaf .
The vascular bundles are conjoint , collateral and closed . The xylem is present on the upper side and phloem on the lower side . Cambium is absent between them .Xylem shows vessels , trancheids , parenchyma and fibres . Phloem shows sieve tubes companion cells and phloem parnechyma .
Each vascular bundle is surrounded by a layer of specialised mesophyll cells that are arranged closely and compactly without intercellular spaces . This layer is called bundle sheath or border parenchyma .The bundle sheath cells divide and grow towards the upper and lower epidermal layers . These are called bundle sheath extensions . They help in the conduction of food materials form the mesophyll to the vascular bundles .