Seagrasses are herbaceous plants (although stiff and hard upright stems and rhizomes do occur), and can be grouped into three main morphological categories with some taxonomic implications.
a. Plants without strap-shaped leaves but with either a pair of petiolate leaves at the rhizome node or two or more leaflets on each of the distal nodes of the erect stem. This category is restricted to Halophila, which has the smallest shoots among seagrasses. Shoots can be less than 1 cm in length, as for H. beccarii and H. minor (Fig. 1B and C), and up to 20 cm long as for H. australis.
b. Shoots with a distinct erect stem and strap-shaped leaves borne at the top of an erect stem. This group includes Thalassia of the Hy-drocharitaceae and all genera of the Cymod-oceaceae (Fig. 1F and I).
c. Plants without visible erect stems, but with strap-shaped leaves derived from the rhizome nodes. Enhalus of the Hydrocharitaceae, the Posidoniaceae and all members of the Zoster-aceae belong to this group (Fig. 1A, D and E). The leaves of some members of Zostera subgenus Zosterella can be as small as 10 cm; while for Enhalus, Posidonia, Zostera subgenus Zostera and Phyllospadix, it is not uncommon for them to reach 1 m or more.
Vegetative organs of all seagrasses have a well-developed creeping rhizome. At each node or at certain nodes there are one or more branched or un-branched roots and a shoot or an erect stem bearing several leaves.
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