Veins are rigid, linear structures. These arise from the petiole and the midrib and traverse the leaf lamina in different directions.
The veins divide and redivide to form the skeletal structure of leaf lamina. Such ramification shows different types of arrangements in different leaves. The ramifications are the veins and the vein let’s while their arrangement is known as venation.
It serves to distribute the water and dissolved mineral salts throughout the expanded lamina and carries the prepared food from it. It gives mechanical strength to lamina helps in conduction.
Angiosperm leaf shows two principal types of venation. When veins and vein lets are irregularly distributed forming a net like reticulate structure, it is called reticulate venation. When they run parallel to each other, it is said to be parallel venation.
Reticulate venation is characteristic of dicotyledons, while the parallel venation is found in monocotyledons. However, there are some exceptions. Among monocotyledons, Smilax, Dioscorea and a few others show reticulate venation and among dicotyledons Calophyllum and a few others show parallel venations.
A. Reticulate venation:
1. Unicostate or pinnate:
The strong nerve that passes from the base of the petiole to the leaf apex is known as midrib or Costa. It bears a large number of laterally placed veins. Further branching of veins is known as vein lets. It resembles the general plan of a feather. Hence, it is called pinnate. Since it possesses a single prominent vein, it is called unicostate. This type of venation is common in dicotyledons like Ficus, mango, etc.
2. Multlcostate or palmate:
In this case, strong veins are more than one. These costae arise from the tip of the petiole and proceed outwards. So, it is called multi costae or palmate type of reticulate venation.
When costa arising at the base of the leaf-blade diverges from one another towards the margin of the leaf, never meeting with each other the venation is called reticulate, palmate and divergent type, e.g., China rose, Castor, Cucumber. But when the costa run in a curved manner from the base and converges towards apex, it is called reticulate, palmate and convergent type, e.g., cinnaon, ca.mphor.
B. Parallel venation:
In this case, the leaf has one or more prominent midribs, which gives off lateral veins that proceed parallel to each other towards the margin or apex of the leaf-blade as in banana, Carina, turmeric etc.
Parallel venation may be unicostate when there is a single prominent midrib or may be multicostate when there is more than one. In multicostate or palmate venation, if prominent veins (Costa) originate from the base of the leaf lamina and spread out or diverge towards the distal end of the leaf, it is known as divergent type e.g. Palm.
If the prominent veins (Costa) coming out from the base of the leaf lamina and converge towards apex, it is called convergent type e.g. grasses, rice, bamboo.
Simple and compound leaf:
A simple leaf consists of a single lamina which may be entire or incised. Such incision doesn’t proceed up to the mid-rib or the petiole. In a compound leaf they are incision of the leaf blade proceeds to the mid-rib or to the petiole so that the leaf lamina gets divided into several segments called leaflets.
Here the midrib is converted into rachis at the base of which there is axillaries bud like that as a simple leaf. There are two types of compound leaves such as pinnate and palmate.
Compound leaf and branch:
A compound leaf looks similar to the branches, yet it is distinguished by the following points:
(1) A compound leaf never bears a terminal bud which is found in case of a branch
(2) A compound leaf, like a simple leaf always bears a bud at its axil (axillary bud) but branches themselves occupy the axillary position of a leaf.
(3) The leaflets of a compound leaf have no axillary buds where as the leaves borne on a branch have axillary buds one for each.
(4) A branch consists of nodes and internodes, while the rachis of a compound leaf is devoid of them.
A. Pinnately compound leaf:
A compound leaf that bears the leaflets on lateral sides of the rachis is called! pinnate. The rachis bears the leaflets either in alternate or opposite manner. It mayb® of the following types:
In this case, the rachis bears the leaflets directly along its two sides as in a feather,] e.g., Cassia, rose, Margosa, etc. When the leaflets are even in number it is said to be paripinnate, e.g., Cassia. But if the number of leaflets is odd, it is said to be imparipinnate, e.g., rose, potato, Clitoria, etc.
The main rachis is branched to give secondary rachis. The leaflets are borne on secondary rachis, e.g., Caesalpinia, Mimosa, Acacia and Albi/.ia etc. All the bipinnate compound leaves are paripinnate.
In this case, the leaf is thrice pinnate. The secondary rachis again bears tertiary rachis that bears the leaflets. Tripinnate compound leaves are imparipinnate, e.g., Moringa,
When the leaf is more than thrice pinnate, it is said to be decompounds as in rot, fennel (Foeniculum vulgare) and Coriandrum, etc.
B. Palmately compound leaf:
In palmately compound leaf, the rachis does not develop at all so that all leaves are articulated to a point on the top of the petiole. The leaflets diverge from common point like the fingers from the palm. According to the number of leaflets, may be of following types:
In this case, a single leaflet is articulated to the winged petiole, e.g., lemon, Oran and other citrus plants.
Two leaflets articulated to the rachis, e.g., Balanites, Bignonia etc.
When three leaflets are articulated to the rachis e.g.,Oxalis, Aegle, etc.
Four leaflets are articulated at the tip of the petiole, e.g., Paris quadrifolia and Marsilea.
(v) Multifoliate or digitate:
Five or more leaflets are articulated to the rachis and look like spreading of fingers from the palm, e.g., Bombax, Gynandropsis, etc.