الفهرس 
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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).