The earliest demonstrable vascular land plants, appearing in Silurian (mid-Ludlovian) time. Their small, leafless axes were usually branched dichotomously in three planes; adventitious and perhaps pseudomonopodial branching also occurred. Sporangia were usually terminal on the main axes. Some terminated in lateral branches, and some were subtended by adventitious branches. The sporangia were globose, almost reniform, elongate-ellipsoidal, or cylindrical; some were columellate and branched. Dehiscence was unspecialized; some opened by an apical slit, and some by a modified distal region. The spores were simple and produced in tetrads. A possible gametophyte plant, associated with Rhynia, was erect, leafless, and terminated by a gametangiophore bearing antheridia and possible archegonia. The basal parts, where known, were rhizomatous, with rhizoids but no true roots. Stomata have been found on some axes and sporangia. The xylem, where known, was centrarch, and consisted of annular elements. Evidence of phloem sieve cells is unconvincing.