It is more strongly illuminated on the upper surface than the lower. This unequal illumination induces a difference in the internal structure between upper and lower sides. A section made at right angle to one of the bigger veins reveals following internal structures.
(1) UPPER EPIDERMIS
This is single layer of cells with a thick cuticle which checks excessive evaporation of water from the surface. It does not contain chloroplasts. Stomata are also usually absent.
(2) LOWER EPIDERMIS
This is also single layer but with a thin cuticle. It bears many stomata, two guard cells, which contain some chloroplasts, and none are present in the epidermal cells. Internal to each stoma a large cavity known as respiratory cavity may be seen. The lower epidermis of the leaf is meant for exchange of gases (oxygen and carbon dioxide) between atmosphere plant body. Excess water also evaporates from the plant body mainly through lower epidermis.
(3) MESOPHYLL
The ground tissue lying between upper epidermis and lower one is known as the mesaphyll. It is differentiated into Paliside parenchyma and spongy Parenchyma.
(a) Paliside parenchyma: It consists of one to two or three layers of elongated more or less cylindrical cells closely packed with their long axes at right angles to the epidermis. The cells contain numerous chloroplasts and manufacture sugar and starch in the presence of sunlight.
(b) Spongy Parenchyma: It consists of oval rounded or irregular cells loosely arranged towards the tower epidermis enclosing numerous large, intercellular spaces and air cavities. They fit closely around the vein or vascular bundle. The cells contain few chloroplasts. Spongy cells help diffusion of gases through the empty spaces left between them. They manufacture sugar and starch to some extent only.
(4) VASCULAR BUNDLES
Each vascular bundle (vein) consists of xylem towards the upper epidermis and phloem towards the lower. Xylem consist of various kinds of vessels (annular and spiral) tracheids, wood fibres and wood Parenchyma. Xylem conducts and distributes water and raw food material to different parts of the leaf blade. Phloem consist of some narrow sieve tubes, companion cells and phloem parenchyma. Phloem carries the prepared food material from leaf blade to the growing and storage regions. Surrounding each vascular bundle there is a compact layer of thin walled cells containing few chloroplasts or none at all. This layer is known as border parenchyma or bundle sheaths. It may extend radially towards upper and lower sides. Frequently sclerenchyma occurs as a sheath complete or incomplete surrounding a bigger bundle or as patches associated with xylem and phloem. Otherwise its distribution in the leaf is somewhat irregular.
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