a division of ancient vascular plants that includes one order—Psilotales. The plants are sometimes considered to be a class (Psilotopsida). The Psilotophyta are small rootless plants that are epiphytic or grow on humus-rich soil and in rock crevices. The underground rhizomatous organs contain fungous hyphae (endotrophic mycorrhiza). The stems are covered by an epidermis with stomata that are structurally similar to the primitive stomata of fossil Rhyniaceae.
The Psilotophyta include two genera, Psilotum and Tmesipteris, which have polymorphous species and are usually combined into the family Psilotaceae. There are two species of Psilotum, each having small scalelike leaves and repeatedly dichotomously branching stems that are 20–100 cm long. Both species are found in the tropics and subtropics of both hemispheres. The genus Tmesipteris also has two species, whose stems measure 5–40 cm long and bear well-developed blades (sporophylls) in the upper portion. The Tmesipteris are found primarily in Australia, Tasmania, New Zealand, and Polynesia.
The synangia that form on the Psilotophyta are bilocular or trilocular. The spores, as they sprout, give rise to underground gametophytes that lack chlorophyll and live saprophytically on fungi. The gametophytes do not differ from the young rhizomatous organs in size and form.
M. E. KIRPICHNIKOV