Evolution of the Leaf

The leaf is the most important organ of a green plant because of its photosynthetic activity. It is very interesting to trace the origin of leaf in the green plants.
The evolution of one-veined leaf (microphyllous) can be explained by assuming that a thorn like outgrowth (E-nation) emerged on the surface of the naked stem. With an increase in size of the out growth, the vascular tissues were also formed for the supply of water and support to the leaf. Another possibility is that a single veined leaf originated by a reduction in size of a part of the leafless branching system of the primitive vascular plant. This is how the leaf of Lycopodium (club mosses) and equistium (horse tail) came into existence.
Many veined leaf (megaphylious) originated much later. These are the evolutionary modifications of the forked branching system in the primitive plants. The first step in the evolution of this leaf was the restriction of forked branches to a single plane. The branching system became flat. The next step in the evolution was filling the space between the branching and the vascular tissue. The leaf so formed looked like the web foot of a duck.