Based on variations in the morphology of the several hundred game-tophytes that have been described, and through extrapolation based on published and personal observations of similarities within genera and families, it is possible to find support for the hypotheses generated in the foregoing discussions of gametophyte types. Most terrestrial Polypodiales (derived ferns) produce annual cordiform gametophytes, whereas most epiphytic Polypodiales produce perennial gametophytes, many of which are also gemmiferous. In the broader context of taxa noted for possessing perennial gametophytes, grammitids
(Polypodiaceae), Elaphoglossaceae, vittarioids (Pteridaceae), and Hymenophyl-laceae, at least 80% of the species grow in epiphytic habitats (Dassler and Farrar, 1997). Thus the correlation appears high between gametophyte type and the habitats where successful gametophyte growth and sporophyte recruitment occur.
In tropical rainforests where epiphytic species abound, one seldom sees a primarily terrestrial species (e.g., Thelypteris, Pteris, etc.) growing as an epiphyte, even as a juvenile sporophyte (Watkins et al., 2006a). This correlation indicates that the reproductive strategies of terrestrial species with annual cordiform gameto-phytes are ill suited for epiphytic habitats. The opposite observation of primarily epiphytic species occurring on terrestrial habitats is more common, but such occurrences are usually in mature, bryophyte dominated sites more similar to epiphytic habitats than to the disturbed soil occurrences of annual cordiform gametophytes. Further, soil-dwelling species of otherwise epiphytic genera (e.g., Trichomanes rigidum and T. osmundoides) retain the gametophyte form of their epiphytic relatives (Farrar, personal observation; Dassler and Farrar, 1997).
Comparisons of physiological functions indicate that adaptations in the gametophytes of terrestrial and epiphytic ferns are not limited to growth form. See Watkins et al. (2007b) and Section 9.3 for a more detailed discussion of this issue. To test the role of gametophyte form in species' migration, Dassler and Farrar (2001) examined the composition of fern floras of continents versus islands in the same floristic zone. They assumed that significant differences in percentage of species with gametophytes of different types would reflect contributions of gametophyte form to successful long-distance migration. Included in the comparison were 32 floras across five continent-island sets (north Pacific, equatorial Pacific, south Pacific, Caribbean, and Indian Ocean). The average total epiphyte percentage was nearly unchanged between continents and islands. Island floras displayed a significant increase in species with gemmiferous gameto-phytes and a decrease in species with perennial gametophytes lacking gemmae (Figure 9.2). These observations might indicate that the ability to multiply and disperse locally via gemmae, following initial introduction by spores, significantly enhances the probability of successful migration when spore dispersal is limited. It should be noted that these estimates did not correct for phylogenetic history, thus the ability to produce gemmae may also lead to greater levels of lineage diversification post-migration. It is inappropriate, however, to conclude from these analyses that perennial but non-gemmiferous gametophytes are mal-adapted to long-distance migration. Neither is it informative to compare across habitat types, but we can speculate that epiphytic species with perennial game-tophytes are better adapted for long-distance migration than they would be if they possessed annual gametophytes.
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