Venation and Phyllotaxy in Leaf

 

The arrangement of veins in the leaf blade or lamina is called venation. Initially this arrangement is directly related to taxonomic significance.  Later on this system has been related with the water relations and has great importance with  ecological research. Venation is of two types namely Reticulate venation and Parallel venation 

Reticulate Venation  is common in all dicot leaves. In this type of venation there is a prominent vein called the midrib in the center .This midrib gives rise to multiple small veins which finally form a network like  mesh structure in the lamina. For Example - Mango

Parallel Venation is found in monocot leaves.  In this type of venation,  all the veins run parallel to each other. Parallel venation is further divided into  two types.

Pinnately Parallel venation  - When,  there is a prominent midrib in the center and multiple   veins  are arisen which run perpendicular or parallel to each other. This is called Pinnately Parallel venation.eg. Banana.

Palmately parallel venation -  Saveral veins  are arisen from the tip of the petiole and they all run parallel to each other and are merged  at the apex. 

In grass they converge at the apex and hence it is called convergent. In Borassus, all the main veins spread out towards the periphery. Hence it is called divergent.

The arrangement of leaves on the stem or the branches is known as phyllotaxy. The  phyllotaxy prevent the gathering of leaves and also  avoid overcrowding of leaves so that    all the leaves receive the sunlight in  great duration   for photosynthesis. The four main types of phyllotaxy are

Alternate phyllotaxy -  In this type the leaves are arranged alternatively on the stem. Examples .Polyalthia.

Opposite Phyllotaxy: In this type of arrangement, two leaves are present at each node and both leaves are located opposite to each other. In Guava, The pairs of leaves  are present  at the  successive nodes and  are found  in the same direction. On the other hand,  two opposite leaves at the node are located  exactly above those at the lower node . It is termed as Opposite superposed Whereas In Calotropis,  one pair of leaves are placed at right angles to the next upper or lower pair of leaves called the Opposite decussate.

Ternate Phyllotaxy - In this type of phyllotaxy,  there are three leaves attached at each node. Example -  Nerium 

Whorled Phyllotaxy -  when more than three leaves are present in a whorl at each node called whorled phyllotaxy. Examples - . Alstonia.