Progymnospermae

(also Progymnospermopsida), a class of fossil plants occupying an intermediate position between sporophytic and spermatophytic plants. The earliest known Progymnospermae have been traced to the Middle Devonian period; the last known representatives are from the Early Carboniferous. The plants varied in size and included trees with thick trunks measuring up to 1.5 m in diameter. A characteristic feature is the combination of reproductive organs typical of Primofilices and vegetative organs whose anatomy is similar to that of Gymnospermae. In 1960 the American paleobotanist C. Beck demonstrated for the first time the organic link between the vegetative and reproductive parts of these plants and isolated a group of plants whose representatives had been classified earlier among the Psilophyta, Primofilices, Cordaitales, and other groups.

The sporangia of the Progymnospermae were homosporous or heterosporous and had no ring; the spores had a triradiate slit. Unlike the ferns, the axes of all the orders of ramification had a similar anatomical structure. The structure of the vascular system, including that of the secondary xylem, was similar to that of the Gymnospermae (Coniferae, Cordaitales). The leaves or leaflike formations were attached in a spiral to the axes of the last order but were often situated, as in the ferns, in one plane. The principal representatives of the Progymnospermae were Archaeopteris, Callixylon, and Svalbardia.