A subdivision or class of * vascular plants containing two living genera, Psilotum and Tmesipteris, that together constitute the order Psilotales. The fossil forms, which are known chiefly from rocks of the Devonian period, are placed in the order Psilophytales. These fossils include Psilophyton and, from the Rhynie chert, Rhynia. The living psilopsids are considered the most primitive of the vascular plants. The sporophyte is dichotomously branching and bears rhizoids rather than true roots. Small leaflike appendages are often present and the vascular tissue consists of tracheids and poorly defined phloem. The spores, which are homosporous, give rise to a subterranean gametophyte. Fertilization of the gametes relies on the presence of a film of water and embryogeny is exoscopic. In some classifications the living and fossil forms are separated into two subdivisions or classes, the Psilotopsida and Psilophytopsida respectively. |
