Describe the anatomy of Dorsiventral leaf
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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