Plants with a regular alternation between a gameto-phytic (sexual) and sporophytic (asexual) phase. Spo-rophytes with leaves, axes and roots (the latter missing in Psilotaceae), and with well-developed conducting tissues. The leaves scale-like, with a single vascular strand, or markedly compound and described as "fronds". Stems protostelic, solenostelic, or dictyostel-ic, sometimes polystelic; some with limited secondary thickening. Sporangia thick- or thin-walled, homospo-rous or heterosporous, borne terminally on an axis or on leaves. Spores trilete (triradiate) or monolete. Game-tophytes autotrophic or mycotrophic. Male gametes (antherozoids) flagellated. Female gametes (egg-cells) borne singly in flask-shaped archegonia.
Psilotatae Lycopodiatae Equisetatae Filicatae
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