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Abstract Seagrasses are submerged marine angiosperms occurring worldwide in shallow coastal waters, except for the most polar latitudes (Short, 1987). Unlike algae, seagrasses are vascular plants having a network of veins to move nutrients and dissolved gases around the plant, they produce flowers, fruits and seeds and like terrestrial grasses, they have separate roots, leaves and underground stems called rhizomes. These structures enable them to form an extensive network below the surface (Tropical Tropics, 1993) All seagrasses show two types of reproductive strategies: vegetative (established) and reproductive (regenerative). Typically, the reproductive phase will dominate under optimal environmental conditions, whereas the vegetative phase is most critical after disturbances or at the extremes of its distribution (Dawes, 1997). Although sexual reproduction is known for all seagrasses, vegetative growth can be the primary means of expansion when sexual reproduction is curtailed or seeds eaten (Dawes, 1997). Seagrasses may have originated from fresh water and estuarine hydrophytic relatives (Arbor, 1920), or from xerophytic salt marsh like plants (Den Hartog, 1970). The idea of gradual evolution from fresh water relatives is attractive, but to date, the only hard evidence consists of xerophytic fossils of presumed seagrass ancestors. Moreover, if seagrass evolved from xerophytic plants that tolerated salt, they then would have to become tolerant of a hydophytic habitat. Whatever the origin of seagrasses, they probably arose in the mid to late Cretaceous (65 to 40 m ybp) (m ybp: million years before present) after angiospenns began to evolve and spread on land in the earlier portion of this period (120 III ybp) (Dawes, 1997). In adapting ~o a submerged habitat, seagrasses have evolved similar hydrophytic features (Dawes, 1997). The morphological similarities between different species of seagrass are evident and include a creeping rhizome and apical meristem, which produce one or more roots and erect short shoots at each node (Dawes, 1997). The roots are adventitious; they range from thick to fibrous in form, have a root cap, and produce root hairs (Dawes, 1997). The rhizome is an underground primary stem, which is usually herbaceous; branching is either sympodial or monopodial, and its morphology is cyl indrical or compressed (Dawes, 1997). |