Modifications of Leaves:

The principal function of leaf is to synthesize food, by the process of photosynthesis.

Besides it also brings about transpiration. However, the leaves may undergo modification to perform special functions as follows-

 

1. Leaf spines:

In some xerophytic plants like Opuntia, the entire leaf gets modified into a small, stiff, pointed structure called spine to check transpiration.

Sometimes only a part of leaf such as stipules, get modified into spines, to protect plants from grazing animals.

e.g., Zizyphus and Acacia

2. Leaf tendrils:

In certain plants having weak stem, entire leaf or a part of it gets modified into an elongated, thin, cylindrical, coiled, wiry, sensitive structure known as tendril. These tendrils help the plant to climb up on some support.

- Wild pea (Lathyrus), entire leaf is tendrillar,

- Sweet pea (Pisum sativum),terminal leaflets are tendrillar,

- Gloriosa only the leaf apex modifies into tendril, and

- Smilax, stipules become tendrillar.

 

3. Leaf hooks:

In Bignonia unguis-cati (Cat's nail), the terminal three leaflets get modified into three stiff curved and pointed hooks which look like cat's nail.

These hooks cling to bark of tree(support) and help the plant for climbing.

Bignonia is an elegant hook-climber.

4. Phyllode:

In some plants, petiole becomes flat, green and leaf like and performs photosynthesis. This is known as phyllode.

For example, in Acacia auriculiformis, the normal leaf is bipinnately compound and falls off soon.

The petiole gets modified into phyllode.

This is xerophytic adaptation to reduce transpiration.

Phyllotaxy:

Phyllotaxy is the arrangement of leaves on the stem and branches in some definite manner.

The main purpose of phyllotaxy is to provide sufficient light to all the leaves. It is of three types :

1. Alternate Phyllotaxy:

In this type, single leaf arises at each node. The leaves arise laterally on the stem or branches. e.g., Sunflower, Mango, etc,

 

2. Opposite Phyllotaxy:

In this type, two leaves arise from each node in opposite direction.

It is of two types :

i) Opposite decussate phyllotaxy:

When one pair of leaf is placed at right angle to next or lower pair of leaf, it is said to be opposite decussate phyllotaxy.

e.g., Calotropis, Ocimum, etc.

 

ii) . Opposite superposed phyllotaxy:

In this type, all the pairs of leaves on the stem are arranged one above the other.

e.g., Jamun, Guava, etc.

 

3. Whorled or verticillate phyllotaxy:

In this type more than two leaves arise from each node and form a whorl around it.

e.g., Nerium.