gogol

Phylogenetically, these known nonvascular plants are off on an evolutionary side branch, quite distinct from the other metaphytan species. The reasons for this are their lack of vascular tissue and the dependence of their sporophyte generation on their gametophytic one. Thus, although there are useful homologies between the green algae and these bryophytes as regards pigments, photosynthetic products, and cell walls, the course of evolution from the ancestral algae was probably independent of that taken by Rhynia and other vascular plants. This could even mean--and it is not unlikely--that land habitats were successfully invaded at least twice by descendents of the green algae. And these two invasions were based on two different kinds of adaptations, notably descendents with and without vascular tissues and their associated patterns of alternation of generations. But again there is a gap. The transitional forms between the green algae and ancestral mosses and liverworts are not in evidence today any more than are those forms between the green algae and vascular plants.

It may be that these gaps can be narrowed, if not closed, by demonstrating conservative or plesiosemic molecular characters. Molecular evolution might reveal changes that indicate important homologies between multicellular green algae and plesiomorphic vascular and nonvascular plants. Of course, a fossil plant like Rhynia will have few proteins in which amino acids can be sequenced, but the lycopods may turn out to be quite informative.