EPIDERMAL TISSUE SYSTEM
It consists of single outermost layer called epidermis which extends over entire surface of plant body. At surface view the cells of the epidermis are irregular in outline but closely fitted together without intercellular spaces. They appear rectangular in transaction. Epidermal cells are parenchymatous in nature with colourless cell sap. In the leaves and young green shoots the epidermis possesses numerous minute openings called stomata. The outer walls of epidermis are often thickened and cutinized. The cutinized layer or cuticle checks evaporation of water. In many plants epidermis bears hairs of different kinds soft, stiff, sharp, stinging, glandular etc. Epidermis functions as protective tissue. It protects plant body against excessive evaporation of water, attacks of herbivorous animals. Parasitic fungi and bacteria and excessive heat or cold.
Outermost layer of root is called epidermal or pilliferous layer.
MESOPHYLL
In dorsiventral leaf, the ground tissue lying between upper epidermis and lower one is known as Mesophyll. It is differentiated into (a) Palisade Parenchyma (b) Spongy Parenchyma.
(a) Palisade Parenchyma: It consists of one to two or three layers of elongated 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, irregular cells loosely arranged towards the lower epidermis enclosing numerous large intercellular spaces and air cavities. They fit closely around the vein or the 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.
APICAL MERISTEM OF STEM APEX
It is composed of mass of small rounded or polygonal cells which are alike and are in a state of division. These microscopic cells form promeristem its cells differentiate into three regions Dermatogen, Periblem and Plerome. The cells of these three regions grow and give rise to primary permanent tissues in the mature portion of the stem. Section further shows on either side a number of outgrowths which arch over the growing apex. These are the young leaves of the bud which cover and protect the tender growing apex of the stem.
Dermatogen is single outermost layer below which lies periblem. It form cortex of stem. Plermoe lies internal to periblem and is central region of stem apex.
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