Simple and compound leaf:
A simple leaf has a single, entire or lobed lamina or blade. When the lamina becomes lobed due to incision, the lobes remain attached to one another by a portion of the lamina.
In case of a compound leaf, the lamina is divided into separate segments, each of which gives an appearance of a small leaf called the leaflet.
Incision of the lamina:
In typical simple leaf, the lamina is not lobed or incised, example Mangifera, Ficus etc. When the margin of a simple leaf is cut into a number of connected lobes or segments it is known as incision of leaf. Incisions of leaf are of two types – pinnate and palmate.
When an unicostate leaf is incised, it is called pinnate incision. They are of three types.
- Pinnatifid: When incisions are less than half way towards the midrib, the leaf is said to be pinnatifid as in Chrysanthemum.
- Pinnatipartite: When the incisions are more than half way towards the midrib, the leaf is said to be pinnatipartite as in Radish.
- Pinnatisect: When the incisions almost reach the midrib, but the segment still remains attached to one another by a narrow strip of the lamina, the leaf is said to be pinnatisect as in Tagetes.
Pinnately compound leaf:
When the incisions almost reach the midrib and the lamina cuts into a number of segments, the leaves become a pinnately compound leaf, example, rose.
When a multicostate leaf is incised into a number of lobes, it is known as palmate incision. Palmate incision is of three types.
- Palmatifid: When the incisions proceed less than half way towards the base of the lamina, it is said to be palmatifid as in cotton.
- Palmatipartite: When the incisions proceed more than half way towards the base of the lamina, the leaf is said to be palmatipartite as in castor.
- Palmatisect: When the incision almost reaches the base of the lamina, the leaf is said to be palmatisect as in Momordica.
Palmately compound leaf:
If the incisions almost reach the base of the lamina completely separating the lobes, the leaf is said to be palmately compound as in Bombax.
Compound leaves are of two types – pinnately compound leaf and palmately compound leaf.
Pinnately compound leaf:
The petiole of the compound leaf is described as rachis. When the leaves are borne on two sides of the rachis like the pinnae of a feather, the leaf is called pinnately compound leaf. Mostly, the leaflets are developed in pairs. Pinnately compound leaves are of the following types:
- Unipinnate: When the leaflets are borne directly on the rachis, the compound leaf is called unipinnate compound. In some cases, the leaflets are of even number since all of them are developed in pairs. Such a unipinnate compound leaf is called paripinnate compound as in tamarind. In some other cases, the leaflets are in odd number since the lower leaflets are arranged in pairs but the terminal leaf remains singly. Such a condition is described as imparipinnate as in rose.
- Bipinnate: When the main rachis give rise to secondary axes and the leaflets are borne on them, the leaf becomes twice pinnate and is described as bipinnate compound leaf. Example Acacia.
- Tripinnate: In this case, the main rachis branches twice to give rise to secondary and tertiary axes and the leaflets are borne on the tertiary axes. Thus, in this case, the leaf is thrice pinnate. Example,
- Decompound: When the leaf is more than thrice pinnate, it is called decompound leaf as in Coriandrum.
Palmately compound leaf:
In a palmately compound leaf, the leaflets are developed from a common point that is from the tip of the rachis and are radiated into different directions like the fingers of a palm. The different types of palmately compound leaf are as follows:
- Unifoliate : When a single leaflet is articulated at the tip of the rachis, it is known as unifoliate palmate compound leaf as in Citrus.
- Bifoliate: When two leaflets are articulated at a point on the rachis, the leaf is said to be bifoliate palmately compound leaf as in Balanites.
- Trifoliate: In this case, three leaflets are developed from a common point of the rachis, as in Oxalis.
- Qudrifoliate: In this case, the numbers of leaflets are four as in Marsilea.
- Digitate: When the number of leaflet in a palmate compound leaf is four, it is known as digitate as in Bombax.
Modification of leaves:
Leaves may be modified to certain special forms in order to perform specific functions. Different types of modifications of leaves are as follows:
A part of the leaf or the whole leaf may be modified into hard pointed structure called spine. In case of date palm, the apex of the lamina is modified into spine whereas in case of Opuntia, the entire leaf along with the small leaves of the axillary bud are modified into a bunch of spines. These spines save the plant from external injury by animals. Spines are also noticed in the apex and margin of the leaf lamina of Argemone.
The whole leaf or part of the leaf may be modified into the slender thread-like coiled structure called tendril. Example, Pisum, Lathyrus,etc. The leaf tendrils help the plants to climb upon the support.
Leaves are sometimes modified into thick membranous structures called scale leaves. The leaves in case of Asparagus and Casuarina give protection to the axillary buds. The scale leaves sometime become fleshy and store food material in them as in onion.
In case of Nepenthes or pitcher plant, the leaf lamina is modified into a flattened sac-like structure called the pitcher. The pitcher is provided with a strong stalk which often coils like a tendril and holds the pitcher upright. The apex of the leaf is modified into a flattened structure which forms the lid of the pitcher. The pitcher captures the insects and digests them to derive nitrogenous food substances from them.
The leaves of Utricularia or bladder wort are very much segmented. Some of these leaf segments are modified into small bladders. Each provided with a trap-door entrance. The trap door allows the small aquatic animals to enter in, but prevents their exit. Inside the bladder, the aquatic animals are digested and are used by the plants as nitrogenous food.