Seagrass rhizomes are usually herbaceous, cylindrical to laterally compressed, and monopodially or irregularly branched; however, in Amphibolis and Thalassodendron (Fig. 1I), the rhizome branches sympodially and becomes woody. Rhizomes are almost always buried in sediment (the exceptions being Thalassodendron spp. and Phyllospadix spp.) and the persistent, fibrous remains of old leaf sheaths usually cover the rhizome of Enhalus and Posidonia (Fig. 1A and E), and to some extent Zostera, Hetero-zostera, Phyllospadix and Halodule. The lengths of rhizome internodes are relatively long in most of the genera, but they are extremely short in Phyllospadix (Barnabas, 1994b). Erect stems resulting from sym-podial branching of rhizomes are present in all genera in the Cymodoceaceae (Fig. 1F-I), some species of Heterozostera (H. nigricaulis and H. chilensis) in the Zosteraceae, and Thalassia and some species of Halophila (Fig. 1B), in the Hydrocharitaceae. Erect stems form at each rhizome node in most genera, but only form at certain rhizome nodes in Amphibolis, Thalassodendron (Fig. 1I), Thalassia and Heterozostera. Among the above-mentioned genera, only Amphibolis has many branched stems, the others have few branches. Erect stems have numerous nodes representing leaf scars and can last for many years. In the genus Halophila, sections Microhalophila, Spinulosae, Americanae (Fig. 1B) and Tricostatae, all have distinct elongated stems but section Halophila (Fig. 1C) has very short erect stems. Erect stems of Cymodocea, Halodule, Thalassodendron, Amphibolis, Heterozostera, Thalassia and section Spinulosae of Halophila are rigid; while those of Syringodium and most of the Halophila species are soft and hegceous.
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