Some of the plants included in this group were previously included in the Rhyniophyta (Rhyniophytina of Banks, 1975). They can be characterized by dichotomously branched, naked aerial axes with terminal sporangia. The aerial axes arise from horizontal, dichotomizing rhizomes that bear rhizoids; no true roots are known. Sporangial shape
varies from ellipsoidal to branched, and some rhyniophyte sporangia appear to have an opening at the tip; others have been described with an abscission layer at the base of the sporangium. When axes are structurally preserved, they contain a small, terete, centrarch conducting strand of S-type conducting elements. Spores are all of the same morphological type, and hence the plants are considered homosporous. Cooksonia (FIG. 8.12), considered by many to represent the first vascular plant, is typical of this group (see section "Other Rhyniophytes"). We will begin, however, with the
plants from the famous Rhynie chert Lagerstätte, as they represent the best-known of the early land plants and have provided so much information about early land plants in their ecosystems, even though a number of them are now placed within the zosterophylls (i.e., Ventarura, Trichopherophyton, early lycophytes (Asteroxylon).
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