The Puzzle of Psilotum

Prior to the recognition of the true complexity of early land plant evolution, all early land plants were placed in a single Division - the Psilophyta or psilophytes. This group is characterized by its lack of true leaves and roots, the presence of underground stems or rhizomes, and erect, dichotomously-branched, photosynthetic stems. While most members of this group were obviously extinct, there did seem to be one living representative - a curious plant called Psilotum:

While Psilotum looks like an early land plant, studies of its sexual life cycle, particularly the gametophyte stage, suggest that it is actually closely related to ferns! It seems probable that Psilotum is a fern that has lost most of the complex features such as leaves and roots. While many texts refer to the plant as a degenerate fern, perhaps it is best considered an example of a vestigial line of fern evolution.

Ferns show some very complex patterns of evolution and one group - the water ferns - have virtually lost their stems. The plants float on water and consist almost entirely of very small leaves with tiny, trailing roots!

The psilophyte concept is no longer widely used, since many botanists now consider the vascular plants to by polyphyletic.