What is a Leaf and Its Type?

The leaf is a flat, green, lateral growth arising from the node of the stem. The wide flat green portion of the leaf is called lamina or leaf blade. 

The thin portion of leaf by which it is joined to stem is called stalk or petiole. The petiole extends into the leaf as the midribs and joins the stems at the leaf base. The thin lines arises from the petiole and midrib and spread across the leaf are called veins. The veins branch into veinlets. The veins provide support to leaf and carry water, minerals and food.

The arrangement of veins on a leaf is called venation. There are two types of venation: Parallel venation and reticulate venation.

1.0Differences Between Parallel and Reticulate Venation

2.0Types of Leaves

Leaves can be classified in various ways. 

Simple Leaf and Compound Leaf

(i) Simple leaf: In a simple leaf, the lamina is not divided into segments, i.e., the lamina is entire or whole. A simple leaf has an axillary bud in its axil. E.g. Mango, banana, banyan. 

(ii) Compound leaf: In a compound leaf, the lamina is divided into segments called leaflets. In these leaves incisions reach upto the midrib or petiole. Due to this each individual leaf looks like a group of small segments called leaflets. e.g. Rose, gram, neem, pea.

The stalk of the compound leaf is called rachis and it bears the axillary bud. Leaflets do not bear axillary buds.

On the Basis of Shape

(i) Needle shaped, e.g., Pine, Onion.

(ii) Oval, e.g., Guava, Apple.

(iii) Heart-shaped, e.g., Peepal.

(iv) Oblong, e.g., Banana.

(v) Circular, e.g., Lotus, Nasturtium.

(vi) Tapering, e.g., Eucalyptus, Ashoka.

On the Basis of Margin

(i) Complete or entire margin, e.g., Peepal.

(ii) Toothed or serrate margin, e.g., China Rose, Rose.

(iii) Wavy margin, e.g., Ashoka, Mango

(iv) Spinous margin, e.g., Prickly Poppy.

On the Basis of Arrangement of Leaves

The arrangement of leaves on the nodes of stem and branches is known as phyllotaxy. It prevents overcrowding and helps in better exposure of leaf to sunlight. It is mainly of three types: Opposite, Alternate and Whorled.

(i) Opposite phyllotaxy: If two leaves are borne one on either side of the same node, the phyllotaxy is said to be opposite, e.g. Tulsi, jasmine, guava.

(ii) Alternate phyllotaxy: If the single leaf is born alternately, one at each node, it is called alternate phyllotaxy. e.g. Rose, peepal, China rose, sunflower.

(iii) Whorled phyllotaxy: If three or more leaves are borne at each node, the arrangement is called whorled. e.g. Kaner, Oleander (Nerium).

3.0Modification of Leaves

Along with the important vital function of leaf, i.e., photosynthesis, leaves of certain plants get modified to perform some additional functions to cope up with the needs of environment. It is of following types:

Leaf Tendril

In some weak stemmed plants either the whole leaf or leaflet or some other parts may be modified into tendril. In sweet pea plant, the terminal leaflets are modified into tendril to climb a support or to expose their leaves to sunlight, e.g. Pea and Lathyrus, Gloriosa (leaf tip), Smilax (stipular tendril).

Spines 

The leaves are modified to hard pointed spines to reduce water loss. They also protect the plant from grazing animals, e.g. Asparagus, Opuntia (cactus), prickly pear, etc.

Phyllode 

In Australian acacia, a very much divided lamina falls off to reduce transpiration. The petiole then becomes expanded into a green, flat, leaf-like photosynthetic structure. It is called the phyllode. It helps in conservation of water and reducing water loss.

Hooks 

In Bignonia, terminal leaflets become modified into hooks for climbing on the support. The hooks give the appearance of the claws of a cat.

Scale Leaves

These are dry, brown, membranous, non-photosynthetic leaves which take a protective role and are present on nodes of aerial stem (e.g., Ruscus, Asparagus) or underground stem (e.g., Ginger).

Pitcher 

In Nepenthes lamina becomes modified into a pitcher to capture insects for supplementing nitrogen supply. The leaf tip forms the lid and the petiole becomes a long, thin, coiled structure supporting the pitcher. The leaf base is green, flattened and photosynthetic.

Bladder 

In Utricularia some of the leaf segments become modified to form a sac-like bladder leaving a small aperture. The insects enter through this opening and are digested by the plant digestive juices.

Venus Flytrap

In case of Venus flytrap, leaf blade is divided into two parts, hinged along the midrib. The two parts get interlocked by marginal teeth and prevent the insect from escaping. e.g. Venus flytrap.

4.0Functions of the Leaf

(i) The leaves on their surfaces carry tiny pores called stomata. Through these pores, plants take in gases for respiration and photosynthesis. The gases produced during these processes are also given out through stomata.

(ii) The process by which plants throw out excess water in the form of vapour is called transpiration. It is done through leaves. 

(iii) Leaves manufacture food for the plant body. The process of manufacturing food by the leaves is called photosynthesis. Leaves require water, carbon dioxide, sunlight and chlorophyll for making food.

(iv) In addition, leaves may be variably modified into spines or tendrils or modified to trap insects.

(v) We eat leaves as a food in some plants like Cabbage, Spinach, Lettuce etc.

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