A leaf is laterally borne out of the stem. It is usually a flattened structure. The leaf develops at the node. It bears a bud in its axil. The axillary bud subsequently develops into a branch. Leaves originate from shoot apical meristem. They are arranged in acropetal order.
Main parts of leaf are: leaf base, petiole and lamina.
The leaf is attached to the stem by the leaf base. It may bear two lateral small leaf-like structures called stipules. In monocot plants, the leaf base expands to form a sheath. The sheath partially or completely covers the stem.
In some leguminous plants, the leaf base may be swollen. The swollen portion is called the pulvinus. The petiole helps to hold the leaf blade to light. Petiole is flexible and thus allows the lamina to flutter in the wind. This helps in bringing fresh air to the leaf surface and has a cooling effect.
The lamina is expanded green portion of leaf. Veins and veinlets are present on the lamina. The arrangement of veins on a leaf is called venation. There are two types of venation, viz. reticulate and parallel.
Reticulate Venation: When the veins and veinlets form a network, this is called reticulate venation. Reticulate venation is usually seen in dicot plants.
Parallel Venation: When the veins are parallel to each other, this is called parallel venation. Parallel venation is usually seen in monocot plants.
Types of Leaves
Simple Leaves: When the lamina is entire or when incised; the incisions do not touch the midrib; then the leaf is called simple leaf, e.g. mango, guava, etc.
Compound Leaves: When the incisions on the lamina reach upto the midrib; breaking it into a number of leaflets, then the leaf is called compound leaf, e.g. neem.
A simple way to differentiate between compound and simple leaves is to look for the axillary bud. The bud is always present in case of a leaf but is absent in case of a leaflet.
There are two types of compound leaves, which are as follows:
Pinnately Compound Leaf: In this case, a number of leaflets are present on the rachis (common axis). The rachis represents the midrib, e.g. neem.
Palmately Compound Leaf: In this case, the leaflets are attached at a common point; at the tip of petiole, e.g. silk cotton.
Phyllotaxy: The arrangement of leaves on the stem or branch is called phyllotaxy. There are three types of phyllotaxy, which are as follows:
Alternate: In this type of leaf arrangement, a single leaf arises at each node in alternate manner, e.g. China rose, mustard, sunflower, etc.
Opposite: In this type of leaf arrangement, a pair of leaves arise at each node. The leaves lie opposite to each other in this case, e.g. Calotropis, guava, etc.
Whorled: In this type of leaf arrangement, more than two leaves arise at a node, e.g. Alstonia.
Modification of Leaves
Tendrils: In some plants, leaves are modified into tendrils to assist in climbing, e.g. pea.
Spines: In some plants, leaves are modified into spines for defence, e.g. cactus. The leaf spine in cacti and in some other xerophytes also helps in reducing water loss by preventing transpiration.
Bulb: In some plants, the leaves are modified into bulb for food storage, e.g. garlic and onion.