| # The Families of Angiosperms |

L. Watson and M.J. Dallwitz

### Zosteraceae Dum.

Habit and leaf form. Aquatic herbs. Perennial; with a basal aggregation of leaves, or without conspicuous aggregations of leaves; rhizomatous (monopodial). Hydrophytic; marine; rooted. Leaves submerged. Leaves alternate; distichous; flat; sessile; sheathing. Leaf sheaths with free margins, or with joined margins. Leaves with blades borne edgewise to the stem, or with blades ‘normally orientated’ (mostly?), or with blades borne edgewise to the stem and with blades ‘normally orientated’ (???); simple. Lamina entire; linear (ribbonlike); parallel-veined; without cross-venules. Leaves ligulate. Axillary scales present. Lamina margins entire. Leaf development ‘graminaceous’.

Leaf anatomy. Stomata absent. The mesophyll without crystals. Foliar vessels absent. Minor leaf veins with phloem transfer cells (3 species of Zostera).

Axial (stem, wood) anatomy. Secondary thickening absent. The axial xylem without vessels.

Root anatomy. Root xylem without vessels.

Reproductive type, pollination. Fertile flowers functionally male and functionally female, or functionally male, or functionally female. Unisexual flowers present. Plants monoecious, or dioecious (Phyllospadix). Pollination by water.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in spikes. The ultimate inflorescence units racemose. Inflorescences axillary (enfolded in the subtending sheath, exserted only when fruiting); flattened spikes or spadices, the flowers on one side — males and females alternating in two ranks in the monoecious species. Flowers ebracteate; ebracteolate; small. Perigone tube absent.

Perianth vestigial (if the marginal outgrowths from the axis — retincacula, which sometimes enclose the stamens — are interpreted as perianth), or absent.

Androecium 1 (though interpretable as two fused members); exclusively of fertile stamens. Stamens 1; with sessile anthers. Anthers dehiscing via longitudinal slits; unilocular; tetrasporangiate (representing fusion of two bisporangiate members?). Microsporogenesis successive. Tapetum amoeboid. Pollen shed as single grains (becoming filamentous and up to 2 mm long when released). Pollen grains lacking exine, and dispersed in the sea as long filaments. Pollen grains nonaperturate; 3-celled.

Gynoecium ostensibly 1 carpelled (but subject to fanciful alternative interpretations). The pistil 1 celled. Gynoecium (?pseudo) monomerous; ostensibly of one carpel; superior. Carpel apically stigmatic; 1 ovuled. Placentation apical. Ovules sessile; pendulous; non-arillate; orthotropous; bitegmic; pseudocrassinucellate. Outer integument not contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating. Synergids pear-shaped. Hypostase present. Endosperm formation nuclear. Embryogeny caryophyllad.

Fruit non-fleshy. The fruiting carpel indehiscent; an achene. Fruit 1 seeded. Seeds non-endospermic. Cotyledons 1. Embryo curved. Testa without phytomelan; membranous.

Seedling. Germination phanerocotylar. Hypocotyl internode present (long, and developing below into a large disk bearing rhizoids). Seedling collar conspicuous (if the hypocotyl disk be so interpreted). Cotyledon hyperphyll elongated; assimilatory; more or less circular in t.s. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral (or rather, virtually absent).

Physiology, phytochemistry. Proanthocyanidins absent. Flavonols absent. Ellagic acid absent.

Geography, cytology. Frigid zone to tropical. Widespread coastal, but mainly extra-tropical, and lacking from West Africa, India, Malaysia and South America. X = 6, 10.

Taxonomy. Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Zosterales. APG III core angiosperms; Superorder Lilianae; non-commelinid Monocot. APG IV Order Alismatales.

Species 18. Genera 3; Heterozostera, Phyllospadix, Zostera.

Illustrations. • Zostera angustifolia: as Z. marina var. angustifolia, Eng. Bot. 1430 (1869). • Phyllospadix scouleri: Engler, Das Pflanzenreich 11 (1907). • Zostera marina: Engler, Das Pflanzenreich 11 (1907). • Le Maout and Decaisne: Zostera marina. • Zostera marina: Eng. Bot. 1429 (1869). • Zostera noltii: Lindley. • Zostera noltii: as Z. nana, Eng. Bot. 1431 (1869).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 8th June 2026. delta-intkey.com’.

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