Electronic Flora of South Australia Family Fact Sheet
Phylum Magnoliophyta – Subphylum Seagrasses – Class Liliopsida – Subclass Alismatidae – Order Potamogetonales
Monoecious or dioecious; marine herbs with creeping perennial rhizomes, rooting at the nodes, forming extensive beds. Roots unbranched. Erect shoots annual (or sometimes possibly perennial) and producing inflorescences. Leaves with a distinct blade and sheathing base; sheath ligulate, auriculate; blade linear, flat with 3–7 (–11) parallel longitudinal veins. Between the veins longitudinal striations, clearly visible in fresh or liquid preserved material, indicate diaphragms separating longitudinal air canals. Transverse vascular strands occupy some of the transverse diaphragms and these are also frequently prominent. Tannin cells absent from the leaves, but present in some other parts of the plant. Squamules present within each leaf sheath. Prophylls present at the base of each shoot. Flowers without a perianth, arranged on one side of a flattened spike (spadix) enveloped within a modified leaf sheath (spathe), the margin of the spadix incurved and usually bearing several intramarginal lobes (retinacules); in monoecious species male and female flowers arranged alternately in 2 longitudinal rows. Male flower of one stamen consisting of 2 free, bilocular, longitudinally dehiscent anthers with a ridge-like connective; pollen filiform; pollination hydrophilous. Female flower of a unilocular ovary with one ovule, a single short style and 2 long stigmatic branches. Fruit an achene.
Taxonomic notes: A family of three genera, known commonly as "eelgrasses", distributed in temperate seas of both hemispheres; two of these genera occur in southern Australia where they are known locally as "garweed".
The Zosteraceae are a clearly circumscribed group. Its members are much reduced anatomically. Lignin is virtually absent (Tomlinson, 1982, p. 423). The highly specialised arrangement of the reproductive organs is quite distinct from that of any other family.
Many instances have been noted of adventitious shoots with horizontal rhizomes arising from mature erect fertile shoots. These adventitious shoots may develop roots, become detached and act as propagules.
References: The Marine Benthic Flora of Southern Australia Part I
Womersley, H.B.S. (31 May, 1984)
The Marine Benthic Flora of Southern Australia
©Board of the Botanic Gardens and State Herbarium, Government of South Australia
KEY TO GENERA OF ZOSTERACEAE
1. Internode in transverse section showing
1. Internode in transverse section showing only 2 vascular bundles in the cortex, erect shoots always developing inflorescences; retinacules triangular to suborbicular.
State Herbarium of South Australia