any of two different groups of ribbonlike aquatic plants. Vallisneria species (family Hydrocharitaceae), also called tape grass, are native to temperate and tropical waters; V. spiralis, often grown in aquariums, is a favourite food of wild ducks. (For its unusual pollination see Alismatales.)

The Zosteraceae, commonly called the eelgrass family, is remarkable for Zostera marina (grass weed or grass wrack), an important tidewater plant whose dried leaves have been used for packing glass articles and for stuffing cushions.