NEET Biology Notes Morphology of Flowering Plants Leaf
It is a lateral, generally flattened structure borne on the stem. It develops at the node and bears a bud in its axil. The axillary bud later develops into a branch. A typical leaf consists of three main parts, i.e. leaf base (by which leaf is attached to the stem), petiole (stalk of the leaf that connects the lamina with the stem or its branch) and lamina orleaf blade (green expanded part of the leaf with veins and veinlets). In some leguminous plants the leaf base may become swollen, which is called pulvinus. The arrangement of veins and veinlets in the lamina of leaf is termed as venation. It may be either reticulate or parallel.
Exceptionally some monocots like Smilax, Dioscorea, Alocasia show reticulate venation and some dicots like Calophyllum, Eiyngium, etc., show parallel venation.A leaf may be either simple (i.e. entire lamina without incision or incision does not reach up to the midrib) or compound (i.e. the incision of the lamina reaches up to the midrib breaking it into a number of leaflets).
The compound leaves may be either pinnately compound (i.e. a number of leaflets are present on a common axis called rachis, which represents the midrib of a leaf as in neem) or palmately compound (i.e. the leaflets are attached at a common point of the tip of petiole as in silk cotton).
Arrangement of Leaves/Phyllotaxy
The pattern of arrangement of leaves on the stem or branch is called phyllotaxy. This is usually of three types:
- In alternate type of phyllotaxy a single leaf arises at each node in alternate manner, e.g. Mangifera indica (mango). Hibiscus rosa sinensis (China rose), Brassica campestris (mustard), Nicotiana tabacum (tobacco).
- In opposite type of phyllotaxy, each node gives rise to two leaves lying opposite to each other,
e.g. Calotropis, guava. Opposite phyllotaxy may be either opposite superimposed (i.e. position of two leaves of each node resembles with the leaves of upper node),
e.g. Eugenia, Quisqualis, Ixora, etc., or opposite decussate, (i.e. leaves of a node are at right angles to the leaves of next node),
e.g. Calotropis procera, Ocimum, etc. It should be noted that in guava (Psidium guavava) both types of arrangements are found.
- In whorled phyllotaxy more than two leaves arise at a node and form a whorl,
e.g. Alstonia, Nerium, Vangueria, etc.
- Leaves are often modified to perform functions other than photosynthesis.