Venation: As the leaves are the food manufacturing organs, they should be supplied with nutrient salts and water, for such purpose profuse conducting system composed of vascular bundles for carrying water and translocation of food materials must ramify in the leaf blade.
These conducting channels form the veins and their branches i.e. veinlets. The leaf blade is a delicate organ and is constantly subjected to wind action.
So a leaf blade or lamina should be provided with ramified mechanical tissues—this purpose is also fulfilled by veinlets. They give mechanical strength and rigidity to the leaf blade.
Therefore, veins and veinlets serve as a vascular i.e. circulatory system as well as skeletal system of the leaf.
The manner in which the veins and veinlets are arranged over the leaf blade or lamina is called venation. Usually, the venation is very clear on the lower surface of the leaves.
In angiosperm leaves, there are two principal types of arrangement of veins and veinlets on the lamina, e.g. (a) reticulate venation and (b) parallel venation. In some gymnosperms (Ginkgo sp.) and in pteridophytes, however, the venation is dichotomous.
I. RETICULATE VENATION —
This type is distinguished by anastomosing network formed by veins and veinlets owing to their irregular repeated branching on the lamina. Reticulate venation is the characteristic of the leaves of dicotyledonous plants.
Reticulate Venation Examples —
Mangifera indica (Anacardiaceae), Artocarpus heterophyllus, Ficus religiosa (Moraceae), Cinnamomum tamala (Lauraceae), Ziziphus mauritiana (Rhamnaceae), Cucurbita maxima (Cucurbitaceae), Hibiscus rosa-sinensis (Malvaceae), Carica papaya (Caricaceae), Ricinus communis (Euphorbiaceae) etc.
Types of reticulate venation —
It is of two types :
(a) Unicostate or Pinnate type — In this type, there is one main rib i.e. strong midrib or costa which branches off giving rise to lateral veins, these again repeatedly branches forming a network, e.g. Mangifera indica (Anacardiaceae), Artocarpus heterophyllus, Ficus religiosa (Moraceae) etc.
(b) Multicostate or Palmate type — In this, there is more than one strong ribs (i.e. costas) which arise from the tip of the petiole ; the strong ribs or costas radiate from the petiole tip and the veins forming a net-like structure.
It is of two forms :
(i) Multicostate convergent – Here more than one costa or strong ribs arise from the tip of the petiole at the base of the lamina and converge at its apex e.g. Cinnamomum tamala (Lauraceae), Ziziphus mauritiana (Rhamnaceae) etc.
(ii) Multicostate divergent – Here more than one costa or strong ribs arising from the base of the lamina at the petiole tip and instead of meeting at the apex diverge from each other, e.g. Cucurbita maxima (Cucurbitaceae), Hibiscus rosa-sinensis (Malvaceae), Carica papaya (Caricaceae), Ricinus communis (Euphorbiaceae) etc.
II. PARALLEL VENATION —
In this type of venation all the veins are disposed in straight lines parallel to one another, and they are laterally connected by transverse veinlets, commonly known as parallel or striate venation. This type of venation is the characteristic of the monocotyledons.
Parallel Venation Examples —
Musa paradisiaca (Musaceae), Canna indica (Cannaceae), all grasses, all fan palms (species of Borassus, Livistonia etc.).
Types of parallel venation —
It is of two types :
(a) Unicostate or Pinnate type – In this case there is a single prominent midrib or costa running from the base to the apex of the lamina, giving off secondary veins parallel to one another which do not branch further e.g. Musa paradisiaca (Musaceae), Canna indica (Cannaceae) etc.
(b) Multicostate or Palmate type – In this case there is more than one prominent ribs (costas) which arise from the base of the leaf blade, and secondary veins i.e. veinlets are formed transversely in parallel manner.
It is of two forms :
(i) Multicostate convergent – When more than one costa or strong ribs run from base of the lamina parallely and converge at the apex, e.g. all grasses.
(ii) Multicostate divergent – In this form all the strong ribs (costas) diverge out from the tip of the petiole towards the margin e.g. all fan palms (species of Borassus, Livistonia etc.).
Chart showing different types of venation :