The stages of the sporoderm morphogenesis ofPsilotum triquetrumare studied by TEM from the newly formed tetraspore stage to the release of the spores. First the tetraspores are overlaid with the “foliated substructure of the exospore”, which consists of some long, thin, anastomosed sheets, and shows a protruding narrow fold at the region of the future aperture; thereafter, sporopollenin without a definite structure gradually accumulates on the substructure thus forming the middle and outer parts of the exospore. Finally, a thin perispore develops close to the surface of the exospore. Either simultaneously or with variable delay, a “pseudo-endospore” fairly thick at the apertural region but very thin elsewhere, appears between the exospore and the spore cell. The endospore itself has not developed when the spore release occurs. This ontogenetical process ofPsilotumis very similar to that of Filices, whereas it obviously differs from that of other Pteridophytes