Leaf | Definition, Structure, Functions & its Modifications
A leaf is a lateral, generally flattened structure borne on the stem. It develops at the node and bears a bud in its axil. It originates from the shoot apical meristem and arranged in an acropetal order of the stem. Leaves are the most important vegetative organ for photosynthesis.
Parts of a Leaf
A typical leaf consists of three parts :
(i) Leaf base : The lowermost (basal) part of the leaf by which the leaf is attached to the node of the stem is called leaf base. Leaf base may bear two lateral small leaf like structures called stipules. In monocots, leaf base expands to form a sheath covering the stem wholly or partially. In some leguminous plants, the leaf base swells and is called pulvinus.
(ii) Petiole : The cylindrical stalk that joins the leaf base with the leaf blade (lamina) is called petiole. It holds the leaf blade above the level of the stem so as to provide sufficient light to the eaf. The long, thin, cylindrical, flexible petiole allows leaf blade to flutter in wind. It thus produces a cooling effect in the leaves by bringing fresh air to the leaf surface.
(iii) Lamina or leaf blade : It is green expanded part of the leaf. The leaf blade is supported by the veins and veinlets. The prominent vein present in the middle of the leaf blade is called midrib. The veins provide rigidity and strength to the leaf blade and also act as a channel for transport of water, minerals and food material. The shape, margin, apex, surface and extent of incision of lamina varies in different leaves.
Venation of Leaf
The lamina or leaf blade contains veins and veinlets. The arrangement of veins and veinlets in the lamina or leaf blade is termed as venation. It gives us a patern in which the veins and veinlets are distributed or arranged in the leaf blade. Venation can occur in two ways
(i) Reticulate venation: The veinlets form a network. It generally occurs in dicots such as peepal, Hibiscus, Luffa etc.
(ii) Parallel venation: The venation where the veins run parallel to each other within a lamina. It generally occurs in monocots e.g., banana.
Types of Leaves
Different types of leaves which exist in nature are :
(I) Simple leaf : The leaf in which the leaf blade is not divided or when incised, the incisions do not touch the midrib. It has bud at the axil of the petiole.
(II) Compound leaf : The leaf in which the lamina or leaf blade is completely broken into distinct leaflets is called compound leaf. The leaf has incisions which reach the midrib. Compound leaf contains a bud at the axil of the petiole but is absent in the axil of leaflets. The compound leaves are of two types:
(a) Pinnately compound leaf : In pinnately compound leaf, the midrib forms a common axis called rachis. A number of leaflets are present on rachis e.g., Neem.
(b) Palmately compound leaf : In palmately compound leaf, the leaflets are attached to a common point i.e., at the tip of the petiole. The tip of the petiole bears all the leaflets in a form of a bunch or cluster e.g., silk cotton.
Phyllotaxy of Leaf
The pattern in which the leaves are arranged on the stem or its branches is caled phyllotaxy. The leaves are arranged in such a way so that all of them get proper sunlight. The leaves can be arranged in three ways i.e., phyllotaxy is of three types:
(i) Alternate phyllotaxy : In alternate phylotaxy. single leaf is present at each node in an alternate fashion. e.g., china rose (shoe flower, mustard, sunflower.
(ii) Opposite phyllotaxy : In opposite phylotaxy, a pair of leaves arise at each node on opposite side. The leaves generally lie opposite to each other at each node e.g., Guava, Calotropis.
(iii) Whorled phyllotaxy : In whorled phyllotaxy, more than two leaves arise at each node and form a whort or a circle. The leaves of one whorl alternate the leaves of the next sucoessive whorl so that all the leaves receive maximum sunlight e.g. Alstonia, Nerium.
Modification of Leaves
Leaves are generally responsible for the process of photosynthesis, transpiration, gaseous exchange etc. But at some places where the conditions are unfavourable, leaves get modify and perform other functions sucn as storage, protection, support, defence etc. Different types of modifications that occur in leaves are :
(i) Leaf tendrils : ln some plants, leaves modify into long, slender, thread-like, sensitive structures called tendrils. They are sensitive to touch and therefore coil around a support to which they come in contact with and help the plant while climbing. Their main function is to provide support to the climbing plants e.g., peas, sweet pea.
(ii) Leaf spines : In some plants such as Aloe, cactus etc. leaves modify into small, sharp-pointed structures which reduce transpiration and protect the plants from browsing animals. The sharp-pointed structures are called leaf spines.
(iii) Storage organ : In some plants such as onion, garlic etc., fleshy leaves store food and hence forms the storage organ in plants.
(iv) Phyllodes : In certain plants such as Australian Acacia the leaves are small and short lived. In these plants the petioles modify to form flat, green-cotoured leaf-ike structure which performs the function of photosynthesis. These are known as phyllodes.
(v) Leaves of certain insectivorous plants such as pitcher plant, venus-fly trap are modified leaves. Pitcher is used to trap insects. These plants obtain nutrients by digesting the insects trapped in the pitcher.