In this article we will discuss about the structure of leafs with the help of diagrams.
Leaves having two surfaces, dorsal and ventral, are called dorsiventral. The upper surface of these leaves is directly illuminated by the sun’s rays. (Fig. 156)
A thin section of such a leaf would show the following structure:
There are two epidermis, upper and lower. The epidermis is usually one-layered with a row of tabular cells, the outer walls of which are properly cuticularised. Stomata are present on the lower epidermis.
II. Ground tissues:
The ground tissue of leaf is called mesophyll tissue. It is differentiated into two types of cells. Towards upper epidermis two or three layers of elongated columnar cells with abundant chloroplasts remain rather compactly arranged more or less at right angles to the epidermis.
They are called palisade parenchyma. Towards the lower epidermis the cells are irregular or isodiametric in shape, and are very loosely arranged with profuse intercellular spaces. They are spongy parenchyma. Next to each stoma on the lower epidermis there is a cavity, called sub-stomatal chamber, which is in communication with the intercellular spaces of the internal tissues.
III. Vascular tissues:
Veins of the leaves are the vascular bundles. The size of the bundle depends on the position one chooses to take in making the section. A section through the midrib shows a vascular bundle with xylem and phloem, xylem with tracheids, vessels, etc., on the outer side and phloem with sieve tubes, etc., on the inner side.
Patches of collenchyma may be present below the upper and lower epidermis for mechanical support. But in a section passing through a smaller vein the bundle remains surrounded by a row of colourless parenchyma cells forming what is known as bundle sheath or border parenchyma.
Both the surfaces of isobilateral leaves are evenly illuminated and thus they show less differentiation of tissues. Stomata are distributed both on the upper and the lower epidermis. Mesophyll is not differentiated into palisade and spongy cells. Patches of sclerenchyma occur here and there for mechanical support.(Fig. 157)