Gametophytes of Psilotum and Tmesipteris grow slowly in axenic culture on a nutrient medium containing minerals and 0.5% glucose. Gametophytes grown in the dark have the typical cylindrical shape and bear antheridia and archegonia. If these gametophytes are grown in the light, they will form apogamous aerial stems from their apices. The apogamous stems have the expected morphology for aerial stems of the two genera. The stems of Psilotum have vascular tissue, stomata, and enations and those of Tmesipteris differ only in having microphylls instead of enations. If the apogamous stems of Psilotum are moved into the dark, their development will shift to that of rhizomes. If the nutrient medium is supplemented with 6-benzylaminopurine, gametophytes will form apogamous rhizomes in the dark. It would appear that the presence or absence of light determines whether the apogamous developments in Psilotum are aerial stems or rhizomes.

Key words: apogamy, Psilotum, Tmesipteris