The leaf is a flat, thin, green lamina or blade made from a soft tissue of thin walled cells supported by a strong network of veins. Leaves are sometimes joined to the stem by a stalk, petiole, which continues into the mid rib (or the main vein). Sometimes there is no leaf-stalk.
In many plants (monocotyledons) the leaf base expands into a sheath which may partially or wholly encircle the stem. In several dicotyledons the leaf base bears lateral appendages called stipules. In many plants of the bean family the leaf-base is swollen called the pulvinus which is responsible for sleep movements.
The arrangement of veins in the lamina is called venation. When veins are irregularly distributed to form a network, the venation is called reticulate (e.g. Ficus and Hibiscus). When veins in the lamina run parallel to one another, the arrangement is called parallel e.g. Lily, banana, bamboo, etc.,
Types of Leaves
Leaves are broadly classified into two groups, simple and compound leaves. In simple leaves the lamina is entire and even if it is incised, the incisions do not reach the midrib. In compound leaves the lamina is divided into a number of segments or leaflets. A leaflet resembles a simple leaf; however, it lacks axillary buds. Compound leaves may be of pinnate or palmate type. In pinnate type of leaves the leaflets are lateral to the midrib or rachis. There are many variations of the pinnate type such as unipinnate of Azadirachta, bipinnate of Delonix and tripinnate of Moringa. In palmate compound leaves the petiole leaves a number of leaflets which resemble the fingers of our palm. Depending upon the number of leaflets the palmate leaves are classified into trifoliate (e.g. methi, fenugreek), quadrifoliate (Marsilea) and multifoliate (Bombax).
Fig : Simple and Compound Leaves
Fig : Tip of Bipinnate Leaf of Delonix
Arrangement of leaves (Phyllotaxy)
The arrangement of leaves on stems shows wide variations among different types of plants. The arrangement of leaves on a stem is called phyllotaxy and it is of considerable help in identifying plants. The three main types of leaf arrangement found in plants are alternate, opposite and whorled.
In alternate form of arrangement a single leaf arises at a node. This arrangement may be in the same plane or in a spiral fashion. E.g. Hibiscus, Mangifera indica.
In the opposite form of arrangement a pair of leaves arise at a node opposite each other e.g. Calotropis.
In whorled condition three or more leaves occur at a node e.g. Nerium.
Fig : Diagrams of Different Leaf Arrangements
The primary function of the leaf is photosynthesis. The water necessary for this process is conveyed through the vessels which run in the vascular bundles branching from the stem and through the petiole and midrib dividing repeatedly to form a network of tiny veins throughout the lamina. The carbon dioxide, which the leaves need for photosynthesis diffuses in, through the stomata. The oxygen needed for respiration also enters through the stomata. Besides this primary function of photosynthesis, leaves in many plants are modified to perform many other specialised functions as well. Leaves are modified into tendrils for climbing as in Pisum, or into spines for defense as in cacti. In Allium, leaves are storage organs for sugars. In Nepenthes, leaves are modified into pitchers for trapping insects. In Australian Acacia plants the leaves are tiny and ephemeral (short-lived). The petioles of these leaves expand, become green and take over the function of photosynthesis.
Certain plants show more than one type of leaf and this phenomenon is called as heterophylly e.g. aquatic plants in running water. The leaves of these plants are of two types. The floating leaves are broad and expanded whereas the leaves submerged in the water are ribbon-shaped or dissected in nature e.g. Sagittaria and Ranunculus.
Fig: Heterophylly in Plants