The roots of seagrasses are adventitious and arise from the lower surface of the rhizomes, generally at the nodes. The external morphology of roots often has characteristic features in different genera, which do not necessarily completely relate to specific types of substrates. For example, Enhalus bears several coarse, soft, unbranched roots with a few short root hairs (Fig. 1A), and grows on muddy substrata. In contrast, both Thalassia and Halophila (Fig. 1B) produce unbranched roots with massive root hairs for penetrating various types of substrata. Roots of Zostera and Heterozostera in the Zosteraceae are always arranged in two groups, each bearing 212 unbranched, slender roots with numerous long root hairs (Fig. 2A, B and J) for growing in muddy to sandy substrata. On the other hand, roots of Phyllospadix also occur in two groups at rhizome nodes, but they are extremely short and covered with dense root hairs, together with robust rhizomes (Fig. 2L-N). Unlike other seagrasses, Phyllospadix can firmly attach to rock surfaces in high-energy environments (Cooper and McRoy, 1988; Barnabas, 1994b). The roots of Posidonia are thick, soft and extensively branched (Fig. 1E), but root hairs are rare (Fig. 2F), and are usually associated with sandy substrata. The Cymodoceaceae including Syringodium, Cymodocea and Halodule have, at each rhizome node, one or more moderately branched roots with few root hairs (Figs. 1F-H and 2D); they usually grow on coral sands. The roots of Amphibolis are wire-like and moderately branched, with no root hairs, while those of Thalassodendron are woody, thick and strong with a shiny black surface, rarely branched, with a few short root hairs only near the root tip (Figs. 1I and 2C and E). In general, Amphibolis and Thalassodendron are associated with firm sedimentary and, sometimes, rocky substrata; the roots of Thalassodendron can penetrate the gaps in hard calcareous reef floors.
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