Generative reproduction is carried out in seagrasses by regular flowering. The majority of seagrass species are perennial and only certain populations of Zostera marina (Keddy and Patriquin, 1978; McMillan, 1983b; Van Lent and Verschuure, 1994; den Hartog, unpublished), Halophila decipiens (McMillan and Soong, 1989; Kuo and Kirkman, 1995; Kenworthy, 2000), and Halophila tricostata (Kuo et al., 1993) have been identified as annuals. The perennial populations may show both generative and vegetative growth, while the annul populations de pend completely on generative growth i.e. recruitment from seeds.
Nine of the 12 seagrass genera are dioecious (although some species of Halophila, e.g. H. decipiens, H. capricorni and H. beccarii are monoecious), in comparison with less than 10% of the entire number of angiosperm genera. Dioecy probably plays an important role in the outcrossing mechanism in seagrasses (McConchie and Knox, 1989a; see also Waycott et al., Chapter 2 and Ackerman, Chapter 4). The Posidoniaceae are exclusively monoecious; in contrast, the Cymodoceaceae are entirely dioecious, while the marine Hydrocharitaceae and the Zoster-aceae contain both monoecious and dioecious genera. All genera of the eurysaline aquatic plants are monoecious, with the exception of Lepilaena, which has monoecious as well as dioecious taxa.
Most aquatic plants retain the floral systems of their terrestrial ancestors, which were not originally adapted to function in water. Some species have acquired floral modifications that allow pollination to function efficiently in aquatic habitats, a phenomenon known as hydrophily. For the reproductive organs, seagrasses exhibit unique structural adaptations for marine submerged pollination. Many floral structures in seagrasses are simplified with a reduced perianth in comparison with terrestrial flowering plants, and these structures may vary with sea-grass groupings. This fact indicates that the affinities between the seagrass families are not at all clear (Tomlinson, 1982). Furthermore, it also reflects the fact that seagrasses have evolved from more than one common ancestor, through different evolutionary pathways, and geological periods (den Hartog, 1970; Larkum and den Hartog, 1989; Philbrick and Les, 1996; Les et al., 1997). Most of their freshwater or terrestrial cousins no longer exist and identification of seagrass evolutionary pathways is quite difficult, if not impossible (but see Chapters 1 & 2).
Floral development and morphology in seagrasses have been reported in several species including Am-phibolis (see McConchie and Knox, 1989a). In A. antarctica, the flowers form on lateral branches while in A. griffithii, they may also develop terminally on an upright branch from the rhizome. Female flowers develop from a pair of primordia, which, in A. griffithii develops three stigmas, while in A. antarctica, may form secondary branches. The ovary wall bears the initials of the grappling apparatus, comprising four comb initials in A. griffithii and a further inner set of horns in A. antarctica.
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