Psilophyta

Psilophyta is a historic division of vascular land plants that once grouped the whisk ferns and their relatives, notably Psilotum and Tmesipteris. Descriptions date to the 19th century, and for much of the 20th century the group was treated as a primitive, seedless division. In contemporary classifications, Psilotum and Tmesipteris are typically placed within the fern lineage as the order Psilotales, within the broader group of monilophytes.

Whisk ferns have a very simple morphology: stems that branch dichotomously and lack true leaves and roots. The photosynthetic shoot is typically free-standing, with subterranean or underground rhizomes. Enations or scale-like appendages may be present on some species, but there are no complex megaphylls. Spore-bearing organs are borne in synangia that develop along the stems.

Reproduction is through spores that are usually homosporous. The sporophyte is the dominant generation; the gametophyte is subterranean and often mycotrophic, living in association with fungi. Fertilization yields a new sporophyte that resembles the mature plant.

Ecology and evolution: Psilophytes occur in tropical and subtropical habitats, often in moist, shaded environments, and sometimes as epiphytes. They are among the earliest-diverging lineages of vascular plants, providing insight into the anatomy of early land plants. Molecular studies generally place Psilotales as a distinct, early-branching lineage within the fern-related portion of the vascular plant phylogeny, rather than with lycophytes.

Fossil record: The early fossil flora includes organisms with dichotomous branching and synangia; genera such as Psilophyton and related rhyniophyte-like plants illuminate the primitive condition of vascular plants in the Silurian–Devonian. The modern whisk ferns retain only a few primitive features, while others have been simplified.