Significance of gametophyte form in tropical, epiphytic ferns

Dassler, Cynthia
Major Professor
Donald R. Farrar
Committee Member
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Fern gametophytes are variable in growth form and longevity. Most are short-lived and cordate in outline. Others are long-lived, ribbon-like or filamentous, and clone-forming, and some produce gametophytic gemmae. Literature, field, and laboratory studies reveal that persistent, clone-forming gametophytes are typical of rainforest epiphytes. Of this group, gametophytes of Elaphoglossaceae and Polypodiaceae lack gemmae, whereas gametophytes of Hymenophyllaceae, Grammitidaceae and Vittariaceae regularly produce gemmae. Correlation of long-lived, clonal gametophyte form with the epiphytic habitat suggests adaptation to the epiphytic habitat. In the epiphytic habitat, bryophytes are a ubiquitous presence, typically covering tree limbs and trunks with a dense mat of plants several centimeters thick. Epiphytic fern gametophytes may have evolved persistent, clonal growth forms to compete with epiphytic bryophytes. Long-lived, clonal growth increases opportunity for gametophytes to find favorable space and/or time for production of sporophytes;In greenhouse cultured bryophyte mats, short-lived, cordate gametophytes grew only in areas devoid of bryophytes, and sporophytes of terrestrial species did not survive to maturity. Gametophytes of Hymenophyllaceae and Vittariaceae grew well inside bryophyte mats, as did sporophytes of all five epiphytic families when transplanted directly to bryophyte covered boards;Studies on epiphytic ferns indicate that they, like terrestrial species, are outbreeders. In gemmiferous epiphytic species, an antheridiogen system results in antheridia formed directly by germinating gemmae when they germinate in the presence of mature gametophytes. In epiphytic habitats, outcrossing between two gametophytes is made difficult by intervening bryophytes. Persistent gametophyte growth can decrease the physical distance between plants, increasing the opportunity for gametophytes to outcross. Gemmae, transported by wind, gravity, animals or water, can also eliminate the separation between gametophytes on different limbs or on different trees. Thus gemmae promote outbreeding when gametophytes are distant, as in long-distance dispersal to islands. On islands, epiphytic gemmiferous species are overrepresented, and nongemmiferous species are underrepresented relative to mainlands.