Banks, Harlan P. Division of Biological Sciences, Cornell University, Ithaca, New York.
Mid-Early-Devonian into Mid-Devonian vascular plants at a higher evolutionary level than Rhyniopsida. Branching was profuse and varied, dichotomous, pseudomonopodial, helical to subopposite and almost whorled, and often trifurcate (see illustration). Vegetative branches were often in a tight helix, terminated by tiny recurved branchlets simulating leaf precursors. The axes were leafless and glabrous or spiny. Fertile branches were trifurcate or dichotomized several times in three planes and terminated in loose or tight clusters of 32–256 fusiform sporangia that dehisced longitudinally. The number of sporangia varied, in part, by abortions or failure of dichotomies. Spores were uniform in Apiculi retusispora or Retusotriletes, depending on preservation. Xylem is known only in Psilophyton, and was centrarch. Tracheids were helical, circular-bordered and scalariform-bordered pitted, and peculiarly multiaperturate. Trimerophyton and Pertica are two other natural genera. Form genera include some species of Dawsonites, Hostinella, and possibly Psilophytites and Psilodendrion. Progymnosperms, ferns, and articulates are derived from this group. See also: Embryobionta; Psilotophyta; Rhyniophyta; Rhyniopsida
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