Fig. 283.—The Sea Sedge
Fig. 284.—The Curved Sedge
Fig. 285.—The Great Sea Rush
We now pass to the peculiar Sea Grasses or Grass Wracks (Zostera) which grow in salt water. They belong to the order Naiadaceæ, and are characterised by cellular leaves with parallel veins, and inconspicuous unisexual or bisexual flowers. The perianth, when present at all, consists of two or four scale-like parts, and the stamens correspond in number with these. The ovary is free, and the carpels, one or more in number, contain each a single ovule. In Zostera the flowers are imperfect, and seem to grow in the slit of the leaf. There are two species, both of which grow in shallow water close to the shore, often in such dense masses that they impede the progress of boats. They have long creeping stems that lie buried in the sand, giving off numerous root-fibres, and send up to the surface slender branches that bear grass-like leaves. The flowers are unisexual, and are arranged in two rows on the same side of a flattened stalk that is enclosed in a sheath formed by short leaves. They have no perianth, the male flowers being composed of a single anther, and the female of a one-celled ovary containing a single ovule, and surmounted by a style with two long stigmas.
There are two species—the Broad-leaved Grass Wrack (Z. marina) with leaves one to three feet long and traversed by three or more parallel veins, and the Dwarf Grass Wrack (Z. nana), the leaves of which are less than a foot long, with veins numbering one to three. There is a variety of the former, however, named Angustifolia, in which the leaves are much narrower than usual, and the veins fewer in number.