gogol

The two metaphytan algal phyla have separate origins. What about the rest of the Metaphyta, the multicellular terrestrial plants? Again, from information given earlier, we see good evidence for homologies in the pigments, photosynthetic products, and cell walls of these plants and the green algae (Chlorophycophyta). The problem now comes down to locating a plesiomorph for the land plants and then seeing if it can be homologized to any of the multicellular, aquatic green algae.

One outstanding candidate for the plesiomorph of the vascular land plants is Rhynia gywnne-vaughani. This is a fossil plant of the Silurian and Devonian periods of the Paleozoic (about 400 × 106 years ago). This plant is described by paleobotanists as a leafless, rootless, branching stem. Part of the stem was prostrate on the ground and from it there extended tufts of slender filaments--not roots--into the ground. Presumably they took up water and minerals. The aerial part of the plant had only one specialized structure at the end of some of the stems, interpreted as sporangia, for the formation of spores. Within the sporangia are the expected tetrads of cells, a characteristic of spores. These structures identify the plant as a sporophyte. Stomata are seen on the stems and this suggests that the stems were photosynthetic. Furthermore, the fossils are so well preserved that a very simple vascular bundle, or stele, can be identified in the center of the stem. Tracheids of the stele are even identifiable as annular. They make up the xylem, and surrounding it is the recognizable phloem.

Except for such plants as the duckweed Lemna, no other vascular plant is as simply organized as Rhynia. In the case of Lemna, there is every reason to think of its simplicity as being due to a reduction of parts from much more complex plants.

Now Rhynia as a plesiomorph poses at least three problems: (1) How did more complex vascular plants, with roots and leaves and gametophyte as well as sporophyte stages, arise from it? (2) What was ancestral to Rhynia? (3) Rhynia maywell be the plesiomorph for the vascular plants, but what about a plesiomorph for the nonvascular mosses and liverworts?