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species like white clover (Trifolium repens), reproduce both through vegetative reproduction and through sexually produced seed (Burdon, 1980).

Agamospermy is less widespread than vegetative reproduction, although it has been reported from at least 30 families of flowering plants (Gustafsson, 1947; Grant, 1981), and it is especially common in grasses and roses. Agamospermous species are often polyploids derived from hybridization between reproductively incompatible progenitors. When they have arisen many times, as in the hawk's beards (Crepis) of western North America (Babcock and Stebbins, 1938) or European blackberries (Rubus; Gustafsson, 1943), the pattern of variation makes it difficult to identify distinct lineages that can be called species. The taxonomic distribution of apogamy in pteridophytes is not well known because of the technical difficulties associated with studying spore development. Nonetheless, Manton (1950) cites examples from at least seven genera of ferns and points out that it has been known for more than a century that apogamy can be experimentally induced in many other groups (Lang, 1898).

When vascular plants reproduce sexually, the reproductive structures may be borne in many different ways. In some pteridophytes, like the club moss Selaginella, and in all seed plants, eggs and sperm are produced by different gametophytes. In other pteridophytes a single gametophyte may produce both eggs and sperm, as in most ferns. Even when eggs and sperm are produced on the same gametophyte, however, zygotes most frequently are formed through union of eggs and sperm from different gametophytes (Soltis and Soltis, 1992). Differences in the time at which male and female reproductive structures form often are reinforced by antheridiogens released by gametophytes in female phase that induce nearby gametophytes to remain in male phase (Döpp, 1959). The antheridiogen system of Cryptogramma crispa, for example, appears to enforce outcrossed reproduction even though individual gametophytes are developmentally capable of producing both eggs and sperm (Parajón et al., 1999).

Eggs and sperm are produced by different gametophytes in flowering plants, but anthers and stigmas most often are borne in a single flower. Despite the apparent opportunity for self-fertilization, zygotes most frequently are formed through the union of eggs and sperm derived from different plants (Barrett and Eckert, 1990). Genetically determined self-incompatibility mechanisms appear to have evolved several times in flowering plants (Holsinger and Steinbachs, 1997), but differences in the time at which pollen is released and stigmas are receptive within a flower and spatial separation between anthers and stigmas promote outcrossing, even in many self-compatible plants (Bertin, 1993; Chang and Rausher, 1998). In some species of flowering plants a polymorphism in stigma height is associated with a complementary polymorphism in anther height, a con-

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