STRUCTURE OF LEAF
Different groups of plants hake different variations in the internal structure or foliage leaves. Hie leaves show different evolutionary adaptations for different environmental conditions. The leaf tissues are divided into three fundamental systems. These are epidermal. mesophyll and vascular tissues.
The leaf tissue is covered on both sides by epidermis. The epidermis is fully activated in leaves. It protects the leaf tissue from excessive loss of \kilter. It also gives some mechanical support. The epidermal cells are living. They are ‘‘ idiom chloroplast except guard cells. ‘Hie epidermal cells of certain lower plants (Ferns) and water plants often contain chloroplast. Epidermal cells are rectangular in shaped. They
have undulating radial walls.Epidermis has cuticular layer on the surface. This cuticle has variable thickness. The Walls of epidermal cells of leaves of the xerophytic plants undergo lignification. Deposition of silica also occurs in grasses and other plants. Epidermis of some plants has some specialized cells. These cells are guard cells, cork cells and silica cells. Some subsidiary cells of guard cells are also present.
Guard cells bound an opening called stomata. Stomata are generally abundant on the losser of the leaf. In certain xerophytic leaves, the epidermis is invaginated to form small chambers. Stomata are developed in these chambers. Epidermis of leaves also have different types of hairs or trichomes..These trichomes may. be unicellular or multicellular.
the epidermis. The palisade tissues consist of elongated cells. They are present at right angle to the epidermis surface. The palisade may consist of more than one layer of cells. There are comparatively shorter Cells in the lower palisade layer. The palisade is more specialized photosynthetic tissues. These cells have more chloroplasts than the cells of the spongy tissues. The leaves are divided into three types on the basis of arrangement of palisade:
- Bifacial leaf: In flattened leaves, the palisade layer is restricted to the upper side. Such leaves are called bifacial or dorsiventral.
- Isolateral leaves: lithe palisade is present on both sides of the spongy tissue, then the leaf is called isolateral.
- Centric leaves: The palisade Forms a continuous ring around the spongy tissue in narrow and cylindrical leaves. Such leaves are called centric leaves
2. Spongy parenchyma: The spongy‘ parenchymas are present at lower side. They are almost isodiametric cells. They have abundant air spaces. The presence of a large number of intercellular spaces allo‘%s the free exchange of gases. These spaces also increase the internal surface area.
(c) Veins (vascular tissues)
The vascular system of leaf is present in the veins. Vascular system is continuous throughout the leaf. The Veins have Anastamosing
connections with each other. The main vein forms the midrib of the leaf. It gives narrow branches.
Each vein consists of a single vascular bundle. Xylem is upper and phloem is lower in position in vein. This vascular bundle is surrounded by a single •layer of parenchyma cells to form bundle sheath. The xylem consists of vessels only. The phloem consists of sieve tubes with companion cells. The phloems are totally absent in the endings of the veins.1
The petioles supply a single •large bundle to leaf. This bundle is arc shaped. The opening of arc is towards the upper side. The arrangement of xylem and phloem in a bundle is the same as in the stem. The ground tissue of the petioles is mainly parenchymatous. Sometimes, it has patches of sclerenclivina or collenchyma tissues. strengthening tissue of the leaves. Collenchyma and sclerenchyma ma are also present in bundle sheaths. They also act strengthening tissues.
The veins are parallel in monocotyledonous leaves. Their bundle sheaths extend to upper and lower epidermis. Some sclereids are also found in mesophyll tissues. Extensive fibers are found in some monocot leaves. Dieot km es form reticulate venation. These veins lbrm network in the leaf.