Submerged or floating fresh-water or marine aquatic plants, or marsh herbs. Stems often elongated, slender, branched, jointed; sometimes short and almost wanting. Leaves very various, alto-gether submerged and translucent, or floating and opaque, or all; radical, sheathing at the base; stipules wanting or interpetiolar, page 746connate and sheathing. Flowers small and inconspicuous, usually green, hermaphrodite or unisexual, variously arranged. Perianth either wanting, or of 3–6 inferior segments valvate in bud. Stamens 1–6, hypogynous; anthers basifixed, erect, 1–2-celled. Ovary superior, of 1–6 distinct or more or less connate carpels, each 1-celled with usually a single erect or pendulous ovule; style long or short, stigma various. Fruit of 1–6 indehiscert nutlets or utricles, rarely drupaceous. Seed solitary, testa mem-branous; albumen wanting; embryo straight or curved, radicle unusually large.

A small order, dispersed over the whole world in marshy places, ponds, lakes, rivers, &c, also including some widely spread marine plants. Genera 16, species estimated at 120. The order cannot be said to have any important properties or uses. With the exception of Lepilœna, which is found elsewhere in Australia alone, all the New Zealand genera have a wide range in both temperate and tropical climates.

A. Flowers hermaphrodite. Perianth of 4–6 herbaceous segments.

Marsh plants with linear radical leaves. Flowers on erect scapes. Perianth-segments 6	1. Triglochin.
Aquatic plants with submerged or floating stems and leaves. Flowers in axillary or terminal spikes. Perianth- segments 4	2. Potamogeton.

B. Flowers unisexual [except in Ruppia). Perianth wanting or minute and hyaline.

* Stems and leaves filiform, submerged.

Flowers hermaphrodite, 2 or 3 on a peduncle greatly elongating in fruit. Perianth wanting. Stamens 2; anthers almost sessile	3. Ruppia.
Flowers unisexual, axillary, nearly sessile. Perianth wanting or female small and hyaline. Stamen 1, fila- ment slender	4. Zannichellia.
Flowers unisexual, axillary, nearly sessile. Perianth of 3 hyaline scales. Stamens 3; anthers sessile, cohering by their backs into a columnar mass	5. LbpilÆna.

** Stems creeping in sand or mud in salt water. Leaves ribbon-like, flat, nerved.

Flowers unisexual, enclosed in the membranous sheathing base of a floral leaf

6. Zostera.