Signature was redacted for privacy.Signature was redacted for privacy.Signature was redacted for privacy. Paul Hinz, committee members, are thanked for their encouragement, support, and suggestions for this work.I especially thank Ming-Ren, Sheau-Ham, Jia-Yuh for everything.This work was supported by the Department of Botany, Iowa State University and by the National Science Council of Taiwan, Republic of China.
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GENERAL INTRODUCTIONAlthough the usefulness of the gametophyte generation in phyletic comparisons among ferns was long neglected (Bower 1923; Holttum 1949), the significance of gametophytes in systematics has since been underscored . Because gametophytes are responsible for both habitat selection and sexual reproduction, analyses of the gametophytic generation must be included in fern biosystematic studies (Windham & Haufler 1986).Research has shown that much of fem genetic variation, which is the crucial element of speciation and evolution, is related to the mating system expressed in the gametophyte generation . In some species, the only remaining generation of the life cycles is the independent gametophyte (Farrar 1985;.A number of studies have shown that gametophyte morphologies vary greatly beyond the standard heart shapes usually depicted in textbooks .These variations include the ability to grow indeterminately and branch so that perennial gametophyte clones of considerable size may be produced. In some families, gametophytes also have the ability to reproduce themselves vegetatively by dispersable gemmae (Farrar 1971;Raine 1994;. Gemma production and/or clonal growth have been observed primarily in epiphytic species . The significance of this morphological diversity to the evolution and systematics of ferns cannot be properly assessed without further knowledge of morphological variation and reproductive strategies of epiphytic species.High genetic load, causing depression of sporophyte production, has been considered to be the primary mechanism promoting outcrossing and thus increasing genetic diversity . However, genetic load of epiphytic ferns has been studied in very few species Ranker 1992a).
2Another factor indirectly affecting the mating system is the hormone (pheromone)antheridiogen, which stimulates antheridia formation and may induce fern spores to germinate in darkness , both responses tending to promote outbreeding. Again, the antheridiogen system in epiphytic ferns has not been extensively investigated and most (the Polypodiaceae) are even thought to exhibit no response to antheridiogen .Most of the variation in gametophyte morphology is among epiphytic species whereas most of the information known on reproductive biology is from research on terrestrial ferns.Epiphytic ferns exist in very different environment from those of terrestrial species. The tropical habitat of gametophytes of epiphytic species is within dense bryophyte mats. In such habitats, interaction between gametophytes via chemical (antheridiogen) or sperm transfer must be significantly hindered relative to gametoph...