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Zosteraceae Dumortier, Anal. Fam. Pl. (1829) 65, 66; nom. cons. Typus: Zoster a L.

Monoecious or dioecious marine plants. Rhizome creeping, herbaceous, monopodial or sympodial;

when monopodial with two vascular bundles in the cortical layer and at each node two or more unbranched roots and a leaf or a prophyllum, with in its axil a short lateral branch bearing a bunch of distichously arranged leaves; roots and rhizomatic leaves alternating; when sympodial (Heterozostera) with 4-10 vascular bundles in the cortical layer and at each node two unbranched roots and an erect stem with distichously arranged leaves and without roots at its nodes. Leaves linear, differentiated into a sheath and a blade with a ligule. Leaf sheath compressed, amplexicaulous, ligulate, either membranous and tubular or open and then auriculate with scarious flaps. Leaf blade linear, with 3-9(-11) parallel nerves and with several accessory bundles between every two of these; nerves connected by perpendicular cross-veins, margin entire, sometimes slightly denticulate or provided with a fringe of uncolored, sclerenchymatic 'fin cells'; tip variable in shape. Generative shoot terminal or lateral, sympodial, erect, consisting of a panicle of rhipidia, but often reduced to a single rhipidium; each rhi-pidium consisting of 2-5 spathes, but sometimes reduced to a single one; peduncle of each spathe partially coalescent with the axis from which it springs or completely free. Spathe consisting of a sheath and a blade; spathal sheath ligulate, open with two more or less overlapping, auriculate flaps, enclosing a sessile or stalked spadix on the dorsal side of which in Zostera and Heterozostera the male flowers (stamens) and female flowers (gynoe-cia) are alternately arranged. Stamens consisting of two free, bilocular, extrorsely length-wise dehiscent, deciduous thecae connected by a reduced ridge-like connective, without a filament; pollen confervoid. Retinacula intramarginal, one beside each stamen, sometimes absent (Zostera subgen. Zostera); on the female spadices of Phyllospadix alternating with the gynoecia. Gynoecium consisting of a superior, horizontally placed, ellipsoid or crescent-shaped ovary with a short thick style and two stigmata of which the distal parts are shed after fertilization; ovule 1, orthotropous, pendulous. Fruit indehiscent, ovoid or ellipsoid with scarious pericarp or else crescent-shaped with the pericarp differentiated into a soft exocarp and a hard fibrous endocarp. Seed 1, ovoid or ellipsoid; embryo macropodous consisting for the larger part of the hypocotyl, which is ventrally grooved; in this groove the short, straight, tubular cotyledon which serves as a sheath for the plumula; primary root usually not developing; endosperm absent. Tannin cells absent.

The family consists of three genera, viz. Zostera, Heterozostera, and Phyllospadix.

There is no doubt about the monophyletic status of the family Zosteraceae. This has already been concluded by Tomlinson (1982) on merely anatomical and morphological grounds. A further confirmation comes from molecular phylogenetic studies of the families of the subclass Alismatidae, using chloro-plast rbcL (Lesetal., 1997; Procaccinietal., 1999a). Les et al. (1997) demonstrated that the Zosteraceae are more closely related to the Potamogetonaceae and the Zannichelliaceae than with the other seagrass groups. These studies, however, didnot support the recognition of Heterozostera tasmanica as representing a distinct genus, but accepted it as a distinct species within the subgenus Zosterella (Les et al., 1997,2002). Based on the matK gene sequence data, Tanakaetal. (2003)also show a similar result. On the other hand, Kato et al. (2003) proposed to divide the Zosteraceae into three genera: Phyllospadix, Zostera and Nanozostera, the genus Nanozostera containing two subgenera, Zosterella and Heterozostera. In this case, by priority Heterozostera, which was established more than 30 years earlier, should be used as the generic name instead of Nanozostera. There is nothing known about the possible ancestors of the family. Originally it was thought that Archeozostera (Koriba and Miki, 1931, 1960) from the Cretaceous of Japan was a protozosterid (den Hartog, 1970), but Kuo et al. (1989) have shown convincingly that Archeozostera is not a seagrass at all, and possibly not even a plant.

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