gogol

Ideally, in a careful phylogenetic analysis, we want to find the actual plants that serve as intermediates between Rhynia and present-day vascular plants. This would fulfill the serial relationship and indirectly establish homologies between Rhynia and such plants as the ferns, conifers, and flowering plants described above. More specifically, we need to show the evolutionary development of roots and leaves. These would be neosemic traits, since both are absent in Rhynia. When both arise, they would then develop aposemically to reach the stages seen in the ferns and the seed plants. Since a stem with a stele is already present in Rhynia, we must look for its aposemic development into forms with more complex tracheids and leaf traces before we reach the level of organization manifested by ferns and seed plants. Similarly, we want to see how sporophytes and gametophytes evolved, aposemically.

Starting with roots and leaves, we find a truly useful series of forms intermediate to Rhynia and other vascular plants. In this series, it should be emphasized, we are in all probability not looking at a series of direct descendants of one plant from the other, but at plants specialized in their own right and representing side branches from some main line of evolution that produced today's vascular plants. These intermediates mark the general direction of evolutionary change, rather than being actual forms that arose sequentially one from the other. Admittedly, though, fossil forms are the most likely place to find them. This series going from Rhynia through Sawdonia, Asteroxylon, to the club mosses or lycopods, is a remarkably complete documentation of neosemic innovation and aposemic changes subsequent to it.

In terms of leaves, Sawdonia has pointed extrusions on its stem, but they are not connected to vascular tissues; hence, they are not true leaves. In Asteroxylon, the lateral extensions of the vascular strands look like leaf traces. They extend from the central stele toward the leaflike scales. And then, in present-day club mosses of the genus Lycopodium, we see true vascularization of the scale-like growths on the stem; they are leaves.

Roots emerged gradually from prostrate stems into struc-tures adapted to root functions, i.e., uptake of water and nutrients, and anchoring of plants. Though we referred to the root as a neosemic structure--it is absent in Rhynia and present in all seed plants--it apparently emerged by the gradual transformation of stem-like parts.

It is worth noting that no structure or function emerges suddenly. Evolution proceeds by the slow accumulation of advantageous mutational changes. Complex structures and their functions are the result of many such changes (thousands or more) and to expect that they would occur simultaneously andproduce a new functional part of an organism that is adaptively significant, is so improbable as to be dismissed. Rather, a slow accumulation of changes, each one beneficial in terms of survival, is the tactic used by evolution. Sometimes complex structures having one function can change their function, and thus selection for one function becomes a preadaptation for another.