Leaves are green, thin flattened lateral outgrowths of the stem. They are borne at the nodes of the stem. Leaves are the chief organs of photosynthesis. The green leaves of the plant are collectively called as foliage of the plant.
Parts of a Leaf
The three main parts of a typical leaf are 1. Leaf base 2. Petiole 3. Lamina
The leaves without stipules are called exstipulate. The main function of the stipule is to protect the leaf in the bud.
Petiole : Petiole connects the lamina with the stem or the branch. A leaf is said to be petiolate when it has a petiole. It is said to besessile when the leaf does not have a petiole.
1.a) Pinnately Reticulate 1.b) Palmately Reticulate
2.a) Pinnately Parallel 2.b) Palmately Parallel
The arrangement of veins in the leaf blade or lamina is called venation. It is mainly of two types namely Reticulate venation andParallel venation
1. Reticulate Venation: This type of venation is common in all dicot leaves. In this type of venation there is a prominent vein called the midrib from which arise many small veins which finally form a net like structure in the lamina. It is of two types
Pinnately reticulate venation : In this type of venation there is only one midrib in the center which forms many lateral branches to form a net work. eg. Mango
2. Parallel Venation: In this type of venation all the veins run parallel to each other. Most of the monocot leaves have parallel venation. It is of two types.
a. Pinnateley Parallel venation : In this type, there is a prominent midrib in the centre. From this arise many veins perpendicularly and run parallel to each other eg. Banana.
b. Palmately parallel venation : In this type several veins arise from the tip of the petiole and they all run parallel to each other and unite at the apex. In grass they converge at the apex and hence it is called convergent. In Borassus (Palmyra) all the main veins spread out towards the periphery. Hence it is called divergent.
Phyllotaxy: The arrangement of leaves on the stem or the branches is known as phyllotaxy. The purpose of phyllotaxy is to avoid overcrowding of leaves so as to expose the leaves maximum to the sunlight for photosynthesis. The four main types of phyllotaxy are
1.Alternate phyllotaxy: In this type the leaves are arranged alternatively in the nodes. There is only one leaf at each node. eg.Polyalthia.
2.Opposite Phyllotaxy: In this type of arrangement two leaves are present at each node, lying opposite to each other. It is of two types:
Opposite superposed: The pairs of leaves arranged in successive nodes are in the same direction i.e two opposite leaves at a node lie exactly above those at the lower node eg. Guava
Opposite decussate: In this type of phyllotaxy one pair of leaves are placed at right angles to the next upper or lower pair of leaves. Eg. Calotropis
3.Ternate Phyllotaxy : In this type there are three leaves attached at each node eg. Nerium
4. Whorled : In this type, more than three leaves are present in a whorl at each node eg. Alamanda.
Simple and compound leaves
Simple Leaf: A leaf is said to be simple in which the leaf blade or lamina is entire. It may be with incision or without incision. e.g.Mango
Compound leaf: Here the lamina is divided in to a number of leaf like lobes called the leaflets. The leaflets are borne on a common axis and they do not bear any axillary buds in their axils. The two types of compound leaf are:
1. Pinnately compound leaves 2. Palmately compound leaves
Pinnately compound leaves
In a pinnately compound leaf, the leaflets are borne on a common axis called the rachis. The leaflets are known as the pinnae. The pinnately compound leaf may be of the type 1. Unipinnate 2. Bipinnate 3. Tripinnate 4. Decompound
1.Unipinnate: In this type the pinnae are borne directly on the rachis. When the number of leaflets is odd, it is said to beimparipinnate eg. Neem .When the number of leaflets is even it is said to be paripinnate eg. Tamarind.
2.Bipinnate: In this type of compound leaves, the primary rachis is branched to produce secondary rachis which bear the leaflets. eg. Acacia.
3.Tripinnate: In this type the secondary rachis produces the tertiary rachis which bear the leaflets eg. Moringa
4.Decompound : When the compound leaf is more than thrice pinnate it is said to be decompound. eg. Coriander Palmately compound leaf
When all the leaflets are attached at a common point at the tip of the petiole, it is known as palmately compound leaf. According to the number of leaflets present the compound leaf may be 1. unifoliate (eg. Lemon) 2. Bifoliate (eg.Zornia diphylla) 3. Trifoliate (eg. Oxalis) 4. quadrifoliate (eg. Marsilia) 5. Multifoliate (eg. Bombax)