Although the ontogeny of fern gametophytes has a long history of morphological investigation, sex expression in ferns has only recently been studied by experimental morphologists and geneti­cists. Most homosporous ferns have bisexual gametophytes whose mitotically derived gametes produce a completely homozygous sporophyte after selfing (Klekowski and Baker, 1966). Wilkie's work (1956), which demonstrated self incompatibility in Pteridium aquilinum, has stimulated the use of fern gametophytes in genetic research. Klekowski and his associates (1968, 1969) used genetic and morphological criteria to characterize pteridophyte gameto­ hytes in terms of their probability for intragametophytic (functionally bisexual) and intergametophytic (functionally unisexual) mating.