The Leaf

• The leaf is a lateral, flattened structure borne on the stem, develops at the node and bears a bud in its axil. The branches develop from axillary bud later. Leaves originate from shoot apical meristems and are set in an acropetal order and leaves are the most important vegetative organs for photosynthesis. 

A characteristic leaf has three main parts:

· Leaf base

· Petiole

· Lamina

Parts of a leaf

• The leaf is attached to the stem by the leaf base and may bear two lateral small leaf like structures called stipules.

• In monocotyledons, the leaf base expands into a sheath covering the stem partially or wholly. In some leguminous plants the leaf base may become swollen, which is called the pulvinus.

• The petiole help hold the blade to light. Long thin flexible petioles allow leaf blades to flutter in wind, thereby cooling the leaf and bringing fresh air to leaf surface.

• The lamina or the leaf blade is the green expanded part of the leaf with veins and veinlets. There is, a middle prominent vein, known as the midrib. Veins provide rigidity to the leaf blade and act as channels of transport for water, minerals and food materials.

• The shape, margin, apex, surface and extent of incision of lamina varies in different leaves.

Venation:

• The organization of veins and the veinlets in the leaf lamina is termed as venation. When the veinlets form a system, the venation is termed as reticulate.

Reticulate venation

• When the veins run parallel to each other within a lamina, the venation is termed as parallel.

Parallel venation

• Leaves of dicotyledonous plants typically have reticulate venation, while parallel venation is the characteristic of most monocotyledons.

Types of Leaves:

• A leaf is said to be simple, if its lamina is full or when incised, the incisions do not touch the midrib. When the incisions of the lamina stretch to the midrib breaking it into a number of leaflets, then it is compound.

• A bud is existing in the axil of petiole of both simple and compound leaves, but not in existence in the axil of leaflets of the compound leaf.

The compound leaves is of two types:

· Pinnately compound leaf

· Palmately compound leaves

• Pinnately compound leaf has a number of leaflets on a common axis, the rachis, which represents the midrib of the leaf. Ex., neem.

Pinnately compound leaf

• Palmately compound leaves, has the leaflets attached at a common point, i.e., at the tip of petiole. Ex., silk cotton.

Palmately compound leaf

Phyllotaxy:

Phyllotaxy is the pattern of organization of leaves on the stem or branch and is of three types –

· Alternate

· Opposite

· Whorled

• In alternate type of phyllotaxy, a single leaf arises at each node in alternate manner. Ex., China rose, and mustard, sun flower plants.

• In opposite type, a pair of leaves arise at each node and lie opposite to each other. Ex., Calotropis and guava plants.

• If more than two leaves arise at a node and form a whorl, it is termed as whorled. Ex., Alstonia.

Different types of phyllotaxy : (a) Alternate (b) Opposite (c) Whorled

Modification of Leaves:

· Leaves are quite frequently modified to do roles other than photosynthesis.

· They are converted into tendrils for climbing ex., peas or into spines for defence ex., cacti.

Modifications of leaf for : (a) support: tendril (b) protection: spines

· The fleshy leaves of onion and garlic store food.

Modifications of leaf for : storage: fleshy leaves

· The leaves are small and short-lived, in plants such as Australian acacia.

· In these plants, the petioles expand, become green and synthesise food.

· Certain insectivorous plants leaves like pitcher plant, venus-fly trap are also modified leaves.

Modification of leaf