In flowering plants many of the diverse characteristics utilized in systematics to determine phylogenetic relationships represent direct adaptations to specific methods of pollination or reproduction. In turn, each of these specific methods can determine to a large extent the total amount of inbreeding and outbreeding occurring and thus control population variability, evolutionary potential, and geographical distribution of the species. These phenomena and influences are well known and have been discussed extensively in the literature. In ferns many of the diverse characteristics utilized in systematics to determine phylogenetic relationships relate to the production of spores and include the size, shape, position and development of the sorus, the sporangia, and the spores contained within. The reproductive mechanisms, however, which determine genetic diversity and evolutionary potential, are held in a free-living independent gametophyte generation, a generation which, with the exception of its morphology, has been largely ignored. The initial modern studies on reproductive biology in ferns were done by Edward J. Klekowski, Jr., in the middle 1960's (Klekowski & Baker, 1966; Klekowski & Lloyd, 1968). Since that time numerous studies have appeared on a variety of species and phenomena by a limited number of workers (Cousens & Horner, 1970; Duckett, 1970, 1972; Ganders, 1972; Holbrook-Walker & Lloyd, 1973; Klekowski, 1969a, 1969b, 1970a, 1970b, 1970c, 1971a, 1971b, 1972a, 1972b, 1973a, 1973b, 1973c, 1973d; Lloyd, 1973a, 1973b, 1974; Lloyd & Klekowski, 1970; Saus, 1973; Schedlbauer & Klekowski, 1972). Homosporous ferns for the most part produce hermaphroditic haploid gametophytes, arising from a single haploid spore by a series of mitotic divisions. As a result, gametes from a single gametophyte will be identical genetically, barring mutation. Self-fertilization, i. e. fusion of sperm and egg from a single gametophyte (intragametophytic selfing) will produce a zygote which is completely homozygous. Thus, in ferns it is possible to produce completely homozygous individuals in a single generation, a condition rarely achieved in flowering plants even after many generations of inbreeding. Due to the complexities of the freeliving gametophyte and the genetic system of ferns, it has been necessary to utilize specific terminology to describe their mating systems. Those terms that will be used in this paper are: (1) intragametophytic selfing: fusion of sperm and egg from a single gametophyte, usually resulting in complete homozygosity of the resultant zygote; (2) intergametophytic selfing: fusion of sperm and egg from different gametophytes but both having arisen from a single sporophyte; this is analogous to inbreeding in flowering plants and results in a zygote with less heterozygosity than the parental sporophyte; (3) intergametophytic crossing: fusion of sperm and egg from different gametophytes, each originating from a