gogol

Asteroxylon shows stem-like structures that clearly look as if they extended into the soil. In the club mosses we see genuine roots. They have the specialized structures and associated functions characteristic of roots. Some place in the line of evolution between Asteroxylon and Lycopodium, real root structures evolved. This appears neosemic because we look at the ends of the series, i.e., forms without true roots and then forms with roots.

The tracheids of the vascular tissue were evolving throughout this sequence, going from annular through helical and scalariform to ones with bordered pits and, eventually, to vessels. Rhynia has annular tracheids. Asteroxylon has helical ones; the lycopods and some ferns have scalariform ones; bordered pits are frequent in seed plants; and vessels characterize flowering plants. Furthermore, it is important to know that in the embryonic development of plants, annular, helical, and scalariform tracheids can all appear, and in that order. It is clearly a case of an evolutionary history persisting in the development of an individual. In the nineteenth century, Ernst Haeckel pronounced this to be the Biogenetic Law. As we said earlier, further work has shown many exceptions to this "law," but here, in tracheid development, is one of its more obvious manifestations.

Also regarding vascular tissues, there is convincing aposemic evidence in the changes shown by the organization of the xylem and phloem, the significance of pith, etc. These details cannot be covered, here, but are amply documented in texts that cover plant anatomy and its evolution. (See references at the end of this chapter.) Finally, it can be noted that certain flowering plants, such as certain cacti, entirely lack vessels. This is another case of simplification through reduction and loss.

Last, we consider the evolution of gametophyte and sporo-phyte stages. In the sporangia of the lycopods there is a gametophyte stage, but whether gametophytes occur in Asteroxylon, Sawdonia, and Rhynia is still conjectural. That is, although there is every reason to believe they occurred in those plants, they have not as yet been identified. A different problem is the two different patterns of occurrence of gametophytes and sporophytes in the ferns and club mosses. In the ferns, there is a separate plant for each stage; in the club mosses, the gametophyte develops within the sporophyte, as it does in the seed plants. In ferns the situation appears to be a dead end--it has gone no further than what we see. The ferns are an example of homosporous plants, i.e., plants that produce only one type of spore. This spore germinates into a gametophyte with male gametes in its antheridia and female gametes in its archegonia.