Gymnosperms

Before considering the two great groups of seed plants, the gymnosperms and the angiosperms, take a moment to speculate on their origin. Think back to a time before woody gymnosperms appeared. In the life-forms studied thus far, liberated spores grow into free-living gametophytes. In many cases, the spores that produce the female gametophytes and those that produce the male gametophytes appear to be the same; they are, thus, called homosporous. The club moss Selaginella, a pteridophyte, is heterosporous, however. Its megaspores give rise to female gametophytes, which remain within their spore walls even after fertilization; and its microspores give rise to male gametophytes, which also remain within their spore walls. They do not become free -living. Yet, the female gametophyte is not surrounded by sporophytic tissue, and thus cannot receive any nourishment from this tissue. Water is still required for the sperm to swim to the egg. Now, take this a step further; envision that the megaspore and resultant female gametophyte remain in the megasporangium on the sporophyte plant and that wind carries sperm to the plant, where the sperm fulfills its mission of fertilization. This sets the stage for the origin of seed plants. As part of this evolutionary trend, microspores and male gametophytes evolve into pollen grains in which germination and sperm development are delayed until transported by wind or insects to the stigma of a flower (that is, to a sporophyte plant). The female gametophyte develops within the ovary of a flower. In this way, the seed plant stage of evolution was accomplished.