Evolution of Seed

We have studied in Selaginella that two types of spores are present. One is smaller in size called micro spore and the other bigger in size called megaspore. This type condition is known as Heterospory. These spores have different functions to perform. Instead of growing into a gametophyte of similar structure, the heteros porous plants produce two different gametophytes. Micro spore grows into a sperm forming gametophyte. The other kind mega spore, grows into egg forming gametophyte. The two kind of spores are formed in two different kinds of sporangia. These like the sporangia of club mosses, horse tails and ferns have become protected as a result of the evolution of various enveloping structure.

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.