Short notes on Epidermal tissue, Mesophyll and Apical Meristem of stem
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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