... microsporophylls, and megasporangia on megasporophylls. Both types of sporophylls usually occur in the same strobilus in Selaginella. In the megasporangium, usually one cell undergoes meiosis to produce four megaspores. In contrast, many cells divide meiotically in the microsporangium, producing hundreds or thousands of spores. The sexual part of the life cycle for heterosporous plants becomes less obvious. The sporophyte plant is increasingly dominant and because the reproductive cycle is speeded up, the next generation can be established more rapidly. This means that evolution itself can be accelerated as changes can be passed on more quickly and the possibility of new life forms appearing becomes greater. Beginning with the primitive, free-sporing, homosporous, monoecious habit, the stages in evolution of heterospory can be summarised as follows: * Gradual decrease in the number of spores in some sporangia. * Increase in size of remaining spores. * Spore content of some sporangia remaining constant in size and number of ...

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