The morphology and ultrastructure of spores of the Early Devonian trimerophytePsilophyton(Dawson) Hueber & Banks

Spores from sporangia of four species of the Early Devonian trimerophyte Psilophyton (Dawson) Hueber & Banks are generally similar in morphology, corresponding to the dispersed spore genera Apiculiretusispora (Streel) Streel and/or Calamospora Schopf, Wilson & Bentall. Some interspecific differences exist in sculptural detail and the shape of the dark area at the juncture of trilete rays. Thin sections of P. forbesii spores reveal a homogeneous sporoderm consisting of two layers similar in electron density, an inner sporoderm and a thinner outer layer representing the ornamentation. The latter detaches in some spores and may represent a thick deposit of tapetal residue, a perispore, a paraexospore, or simply a less well organized layer of sporoderm. Psilophyton spores can be distinguished from the spores of many, but not all, Early Devonian vascular plants for which in situ spores are known.