5.4.3 Anatomy ofLeaf
Anatomically there are two types of leaves:
1. Dorsiventral Leaf:
In this type of leaf the mesophyll tissue is differentiated into palisade parenchyma and spongy parenchyma.
It is very common in dicotyledonous plants.
The leaves are commonly horizontal in orientation with distinct upper and lower surfaces.
The upper surface which faces the sun is darker than the lower surface.
T.S. of Typical dicot leaf shows :
i) Upper epidermis:
It consists of a single layer of tightly packed rectangular, barrel shaped, parenchymatous cells which are devoid of
chloroplast.
A distinct layer of cuticle lies on the outside of the epidermis.
Stomata are generally absent.
ii) Mesophyll:
Between upper and lower epidermis, there is chloroplast-containing photosynthetic tissue called mesophyll.
Mesophyll is differentiated into palisade and spongy tissue.
Palisade parenchyma:
It is present below upper epidermis and consists of closely packed elongated cells.
The cells contain abundant chloroplasts and help in photosynthesis.
Spongy parenchyma:
It is present below palisade tissue and consists of loosely arranged irregularly shaped cells with intercellular spaces.
The spongy parenchyma cells contain chloroplast and are in contact with atmosphere through stomata.
iii) Vascular system:
It is made up of a number of vascular bundles of varying size depending upon the venation.
Each vascular bundle is surrounded by a thin layer of parenchymatous cells called bundle sheath.
Vascular bundles are conjoint, collateral and closed.
Each vascular bundle has xylem towards upper epidermis and phloem towards lower epidermis.
Cambium is absent in leaf, so there is no secondary growth in the leaf.
iv) Lower epidermis:
The lower epidermis consists of a single layer of compactly arranged rectangular, parenchymatous cells.
A thin layer of cuticle is also present. The lower epidermis contains a large number of microscpic pores called stomata.
There is an air-space called sub-stomatal chamber at each stoma.
2. Isobilateral Leaf
In this type of leaf both the surfaces are equally illuminated as both the surfaces can face the sun, and show similar
structure.
The two surfaces are equally green. Generally monocotylednous plants have isobilateral leaves.
A typical monocot leaf resembles a dicot leaf in its anatomical structure. However, it shows some differences such as
presence of stomata on both the surfaces and mesophyll is not differentiated into palisade and spongy tissue.
