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) ia called petiole. It holds the leaf blade above the level of the stem so as to provide sufficient light to the leaf. 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 laef 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 the incision of lamina varies in different leaves.
Venation:- The arrangement of veins and veinlets in the lamina or leaf blade is termed as venation. It occurs 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.
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.
(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. Abud is present in the axil of petiole in both simple and compound leaves.
The compound leaves are two types:-
(a) Pinnately compound leaf:- In pinnately compound leaf, the midrib forms a common axil called rachis. A number of leaflets are present on rachis e.g. Neem.
(b) 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:- The pattern in which the leaves are arranged on the stem or its branches is called phyllotaxy.
Phyllotaxy is of three types:-
(i) Alternate phyllotaxy:- In alternate phyllotaxy, single leaf is present at each node in an alternate fashion. e.g. china rose (shoe flower), mustard, sunflower.
(ii) Opposite phyllotaxy:- In opposite phyllotaxy, 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 whorl or a circle. The leaves of one whorl alternate the leaves of the next successive whorl so that all the leaves receive maximum sunlight e.g. Alstonia, Nerium.
Modification of Leaves:-
At some places where the conditions are unfavorable, leaves get modify and perform other functions other than photosynthesis. Different types of nodifications in leaves are:
(i) Leaf tendrils: In some plants, leaves modify into long slender, thread like, sensitive structures called tendrils. They help the plant in climbing. E.g., peas, sweet pea
(ii) Leaf spines: In some plants such as aloe, cactus, etc leaves modify into small, sharp- structures called leaf spines.
(iii) Storage organ: In onion, garlic, etc., fleshy leaves store food.
(iv) Phyllodes: In plants such as Australian Acacia the petioles modify to form flate, green- colored leaf- like structure known as phyllodes, which performs the function of photosynthesis.
(v) Leaves of certain insectivorous plants such as pitcher plant, venus- fly trap are modified leaves. Pithcher is used to trap insects. These plants obtain nutrients by digesting the insects trapped in the pitcher.
