Epidermis in the leaf is a single layer of cells fitting closely together with no air spaces between them. The epidermis may secrete a continuous waxy layer known as the cuticle which reduces evaporation. The epidermis helps to maintain the shape of the leaf; protects the inner cells from bacteria, fungi and mechanical damage and reduces evaporation. The epidermal cells, except the guard cells of stomata, do not usually contain chloroplasts and are transparent so that sunlight can pass through the cells below which it contains chloroplasts.
The palisade layer of the leaf consists of one or more rows of tall cylindrical cells, with narrow air spaces between them. Photosynthesis occurs in this layer. There are many chloroplasts in the cytoplasm lining the walls. Lying immediately below the epidermis, the palisade cells receive and absorb most of the sunlight.
Below the palisade layer is the spongy layer. The cells in this region do not fit closely together, and large air spaces communicate with each other and through the stomata, with the atmosphere, thus allowing air to circulate in them. The palisade layer and spongy layer together constitute the mesophyll.
Stomata are opening in the epidermis and usually, they are abundant on the lower side of the leaf. They are formed between two guard cells which according to their internal pressure or turgor, can increase or decrease the size of the stomatal opening or close them completely.
Vascular bundles traverse the spongy parenchyma. Each bundle consists of xylem and phloem and is surrounded by a parenchymatous sheath of cells.
Fig: T.S of a Dicot Leaf
Anatomical differences between monocot and dicot leaves
- Leaves in dicots are called dorsiventral as they have a distinct dorsal (upper, facing sun) and ventral (lower) surfaces. In monocots, the leaves are referred to as isobilateral as both the sides are similar in structure. The leaves of monocots are not horizontally placed as in dicots, but are vertically placed. In such leaves, the palisade is present internal to both the epidermal surfaces.
- The stomata are more abundant on the lower surface of the dicot leaves. In monocot leaves, the stomata are equally distributed on the two surfaces.
The epidermis is a layer of cells without a cuticle. The younger regions, particularly those with root hairs, permit the uptake of water and solutes.
The root cortex consists of large thin-walled cells with air spaces between them. The cortical cells store food material and the innermost layer of cells may regulate the inward passage of water and dissolved substances.
Vascular tissue is in the centre of the root. Initially the phloem strands lie between the radial arms of the central xylem. The branches that form the lateral roots grow from this region and force their way through the cortex, bursting through the outer layer to reach the soil. The centrally placed vascular tissue wall adapts the root to the strain that it is likely to experience along its length.
Fig: T.S. of a Dicot Root
A comparison of the dicot and monocot root structure shows more similarities than differences. In both types the epidermis, root hairs, cortex, endodermis and pericycle show similar structural organisation; in both types, the vascular cylinder consists of xylem and phloem which occur in a ring alternating with each other (radial, arrangement).
In monocot roots the xylem and phloem bundles are numerous. In monocot roots the xylem elements of bundles do not meet in the centre and the centre of the root is occupied by parenchymatous cells. This is called pith which is absent in dicot roots. In monocot roots cambium is not formed and therefore no secondary growth takes place.