The leaf is the green flattened structure which develops from the node of the stem or branch of a plant. It is regarded as the most important vegetative organ since most of the vital activities of the plant are performed by it.
The leaves are always developed in an acropetal succession. A typical leaf mainly consists of three parts – leaf base, petiole and lamina.
The part of the leaf with which it is attached to the stem is called the leaf base. Usually, the leaf base is slightly swollen, but when it is distinctly swollen, it is said to be pulvinus.
In monocotyledonous plants, the leaf base is expanded to a sheath which completely or partially covers the stem. In banana, the so-called stem consists of the leaf sheaths. In many dicotyledonous plant, the leaf bases give rise to lateral outgrowths known as stipules.
Stipules are small outgrowths developed from the leaf bases. When a stipule falls off before the unfolding of the lamina, it is called caducous, but when it falls off soon after the unfolding of the leaf, it is known deciduous. If the stipule persists as long as the lamina remains on the stem, it is known as persistent.
The small stipules present at the base of the leaflet of a compound leaf are called stipels. Example Dolichos. When the stipule is present, the leaf is said to be stipulate but when it is absent, the leaf is said to be exstipulate. On the basis of the shape, size and their relative position, stipules may be described as follows:
- Free lateral stipules: When two stipules are developed laterally on the leaf base and they are not fused with each other, they are known as free lateral stipules. Usually, they are small, narrow, and green in color. Examples, cotton, lady’s finger, Hibiscus.
- Adnate stipules: When two lateral stipules are fused with the petioles up to a certain length and then become free at their apex, they are known as adnate stipules. They usually give a winged appearance to the petioles. Examples, rose, groundnut.
- Interpetiolar stipules: These are the two stipules that occupy the position in between the petioles of two opposite leaves. Each stipule is a fused product of two stipules, one from each of the two opposite leaves. Example, Ixora.
- Intrapetiolar stipules: When the stipules lies towards the inner side of the petiole and just face the stem, it is called the intrapetiolar stipules. Such a stipule is formed due to fusion of two lateral stipules at the axillary region of a leaf. Example, Ervatamia.
- Ochreate stipules: When the stipules fuse together and form a hollow tube covering the internode of a stem up to a certain length, they are known as ochreate stipules, example, Polygonum.
Sometimes, the stipules are modified into different forms and serve some special functions. On the basis of this, they have been classified into the following types.
- Foliaceous stipules: When the stipules are comparatively large and flattened like a leaf, they are known foliaceous stipules. Examples, Pisum, Lathyrus etc.
- Bud scale stipules: These are comparatively large which enclose the bud and give protection to it. When the leaves unfold, these stipules fall off, example, jackfruit.
- Spinous stipules: Two lateral stipules are sometime modified into two hard pointed structures called spines. These spinous stipules protect the leaf from the attack of animals. Examples, Zizyphus, Acacia.
- Tendrillar stipules: The lateral stipules may be modified to long slender and coiled structure called tendrils. The tendrillar stipules help the plant to climb upon the support as in Smilax. In this case, the tendrils are developed into two lateral scaly stipules, which are fused with the petioles.
The stalk of the leaf is called petiole. In many cases, the petiole of the leaf is not distinct and such a leaf is called sessile. When the petiole is present, the leaf is said to be petiolate or stalked.
In case of Eichhornia, the petiole is modified to a swollen and spongy structure called the swollen petiole. The petiole of Citrus forms a vein-like structure and is called the winged petiole. In Acacia moniliformis, the petiole is modified to a flattened leaf-like structure known as phyllode. In Clematis, petiole may become elongated and functions like a tendril. Such a petiole is called a tendrillar petiole.
Lamina or leaf blade:
The green and expanded portion of the leaf beyond the petiole is called the lamina. The petiole extends into the median region of the lamina and functions as the strong main vein called the midrib. From the midrib develop a number lateral veins which ramify in the lamina.
Shape of the leaf:
The general outline or shape of the leaf may be of following types.
- Linear: It is long, narrow and flat with more or less parallel edges. Example, Cyperus.
- Lanceolate: It is like a lance, broad at the middle and tapering at both the ends. Example, bamboo.
- Oval or elliptical: It has the shape of an ellipse. Examples Vinca, guava.
- Ovate: It is egg shaped, broader at the base than the apex. Example, Hibiscus.
- Obovate: It is broad at the apex and narrow at the base, inversely egg shaped. Example, Anacardium.
- Acicular: It is needle like, long and narrow and cylindrical. Example, Pinus.
- Cordate: It is heart shaped. Example, Piper.
- Obcordate: It is inversely heart shaped. Example, Oxalis.
- Oblique: The two halves of the lamina are unequal. Example, Azadirachta.
- Oblong: It is long and wide with parallel margin and rounded both at apex and base. Example, banana.
- Spatulate: It is like a spatula, broad and rounded at the top and narrow at the base. Example, Duranta.
- Sagittate: It is like an arrowhead. Example, Sagittaria.
- Cuneate: It is wedge shaped. Example, Pistia.
- Hastate: It is like a arrowhead but lower lobe directed outward. Example, Ipomoea reptans.
- Lyrate: It is like lyre, with a large terminal lobe and smaller lateral lobes. Example, Radish.
- Pedate: It is divided into lobes which spread like the claw of a bird. Example, Vitis.
- Cylindrical: It is like a cylinder. Example, onion.
- Rotund: It is more or less circular. Example, lotus.
- Reniform: It is kidney shaped. Example, Hydrocotyle.