Sporophytes arise in most plants from zygotes formed by the union of two gametes. In approximately 10% of ferns this is never the case (Walker, 1984). Although gametophytes, and often functional antheridia, are produced, the sporophytes in these plants arise most frequently from somatic cells located behind the apical meristem. This is the tissue that traditionally generates archegonia in ferns, and in some of these taxa (e.g., Pteris cretica, Laird and Sheffield, 1986) archegonia still form, but do not function. The names given to the process are many and varied, and include agamospory (Walker, 1985), apomixis (e.g., Lovis, 1977), and apogamy. Detailed discussion of the terms would not be helpful to our theme and, as with the definitions of alternation of generations, their use depends heavily upon which definition you choose. Agamospory and apomixis are the terms favored by workers who are concerned with the process that generates the spores of such ferns (e.g., see Gastony and Haufler, 1976; Gastony and Windham, 1989; and references therein). If one accepts the simplest definition of apogamy - the production of a sporophyte without sexual fusion (Sheffield and Bell, 1987), it makes it easier to start to understand the significance of this method of alternation.
There are essentially two types of apogamy. One is that routinely shown in the life cycle of the ferns referred to above, the other is an induced process -sometimes called facultative or induced apogamy (see Raghavan, 1989). The latter is of interest both as a tool for plant breeders and propagators (e.g., Martin et al., 2006) and as confirmation that the environment can have a powerful influence on the alternation of generations (this will be a recurrent theme throughout the chapter). It does, however, generate plants that have double the number of chromosomes usual for the species, and while it may be of horticultural significance, it is doubtful that it plays a role in natural populations.
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