a subclass of the oldest extinct ferns, which existed from the Early Devonian to the Early Permian. Primofilices, a complex and apparently heterogeneous group, is possibly connected with rhynophytes and psilophytes through primitive representatives. The oldest species reveal a close relationship to early Progymnospermae; hence the division of these groups is conventional.
Some Primofilices externally resembled ordinary ferns, so that it is impossible to differentiate them according to imprints. Others were constructed on the basis of their type of complexly branching three-dimensional system of axes. There were both homosporous and heterosporous forms. The sporangia had a unistratal or multistratal structure, sometimes with a compound ring. The spores had a three-rayed fissure. The anatomical structures of the axes and leaflike formations were extremely varied. In some Primofilices the secondary wood was highly developed. The stems had several isolated steles or only one. In cross section the steles were of various shapes—rounded, elliptic, cruciform, H-shaped, and star-shaped. The principal representatives were Cladoxylon, Zygopteris, and Botryopteris.
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