Large leaves having dichotomously divided veins and veinlets with an expanded leaf blade or lamina are known as megaphylls which are characteristic form of ferns and seed plants. It is suggested that evolution of megaphylls started from a dichotomous branching system in some primitive Fern like plants approximately 350 million years ago.
Steps involved in Evolution of Megaphyll:
It is assumed that evolution of a megaphyll included series of steps which are as follows:
Overtopping: The dichotomously branched aerial portion of the stem showed unequal branching. Some branches remain short others grew in different planes. Such an unequal development of various branches is called overtopping.
Planation: Next there was arrangement of unequal dichotomies (branches) in one plane. this process is termed as planation.
Webbing: In this process, the space between the overtopped dichotomous branches was occupied by a sheet of parenchyma cells which connected these branches forming a flat lamina or leaf blade type of structure.
Fusion: This was followed by the fusion of the vascular strands to form net or reticulate venation pattern.
The process of evolution of leaf was very slow and gradual which completes in more than 15-20 million years.