Silurian-Devonian Terrestrial Algae and Problematica Modern embryophytes embryophytes attest to the evolutionary proliferation of a single lineage of green algae that established a megascopic terrestrial flora. However, it is likely that the invasion of land was attempted by several groups, most of which were ultimately unsuccessful. We leave our phylogenetic framework for a moment to look at some of the early land plants that have boggled attempts at evolutionary classification. Some of these plants are thalloid much like early liverworts, hornworts and mosses might have been. Some have tubes, but their tubes are unlike those of any living organism.
Reproductive structures are rare and confusing.
However, it remains a paleontological truth that fossils belonging to entirely extinct lineages are the most difficult to understand. Cooksonia is characterized by small, slender axes that branch dichotomously . In fertile specimens, sporangia terminate each branch tip. Rooting structures or rhizomes have never been found, probably because most Cooksonia specimens are allochthonous , having been broken off and transported before burial. A few lucky specimens contain a suspicious dark trace in the center of the axis from which a few poorly-preserved conducting tubes have been isolated.