Leaves develop at the node of a stem and are generally arranged in an acropetal order. A typical leaf has three main parts: leaf base, petiole and lamina. The leaf is attached to the stem by the leaf base and may bear leaf-like structures called stipules. In some leguminous plants, the swollen leaf base is known as the pulvinus. The petiole tilts the leaf towards the direction of light. The lamina is the expanded green portion of a leaf, which has a prominent vein running through its centre called the midrib.
The arrangement of veins and veinlets in a lamina is called venation. Venation is of two types, when the veins within the lamina form a network it is called reticulate venation which is a characteristic of dicots. When the veins run parallel to each other, it is called parallel venation which is characteristic of monocots. The veins help to transport water, minerals and food and provide rigidity to the lamina. Based on the structure of the lamina, leaves are classified as simple and compound.
The lamina of a simple leaf is usually entire; the incisions reach the midrib and break it into several leaflets in compound leaves. Compound leaves are further classified into pinnately and palmately compound leaves. In pinnately compound leaves, there are several leaflets on the common axis or the rachis. In palmately compound leaves, all the leaflets are attached to the tip of the petiole.
The leaves are also classified as opposite, alternate or whorled on the basis of their pattern of arrangement on a stem or a branch, called phyllotaxy. In opposite phyllotaxy, two leaves at one node are opposite to each other, in alternate phyllotaxy, only one leaf at each node and the leaf at the next node is in the opposite direction. While in whorled phyllotaxy, two or more leaves appear at a node forming a whorl.
Leaves are modified to perform functions other than photosynthesis. The fleshy leaves of onion and garlic store food, petioles expand and turn green to synthesise food in Australian acacia, leaves capture tiny insects for food in the Venus-fly trap, and the leaves are modified into spines for protection in cacti.