1. Flowers in spikes, hermaphrodite or polygamous, without a perianth, but the stamens sometimes provided with a sepal-like connective. 2 Flowers solitary or in cymes, unisexual. 5 2. Spikes with a flat axis, at the time of flowering enclosed in the sheaths of the uppermost leaves. Stamen 1. Pollen-grains filiform. Carpel 1. Stigmas 2, on a short style. Embryo with a very large radicle and a tail-like cotyledon. Submerged marine plants.--Species 2. North and South Africa and Madagascar. Used for stuffing and as packing material. “Grass-wrack.” [Tribe ZOSTEREAE.] =Zostera= L. Spikes with a cylindrical axis, at the time of flowering not enclosed in the sheaths of the uppermost leaves. Stamens 2-4. Stigma 1, undivided or many-parted. 3 3. Spikes compound, submerged. Spikelets shorter than their bracts. Flowers polygamous. Stamens 3, rarely 4. Pollen-grains filiform. Carpel 1. Stigma divided (or provided with narrow appendages). Embryo with a very large radicle and a straight cotyledon resembling the leaves of the plumule. Marine plants.--Species 1. Mediterranean Sea. The leaves are used for packing and thatching, and also in medicine. [Tribe POSIDONIEAE.] =Posidonia= Koen. Spikes simple, above the water. Flowers hermaphrodite. Pollen-grains globular or bent. Carpels usually 4. Stigma simple, more or less peltate. Embryo with a curved cotyledon. [Tribe POTAMOGETONEAE.] 4 4. Spikes two-flowered. Stamens 2. Anthers with a very short appendage and kidney-shaped cells opening outwards. Pollen-grains bent. Fruit stalked. Embryo with a very thick radicle. Salt-water plants. Leaves subulate.--Species 1. =Ruppia= L. Spikes several-flowered. Stamens 4. Anthers with a sepal-like appendage and straight cells opening laterally. Pollen-grains globular. Fruit sessile. Embryo with a slightly thickened radicle.--Species 20. Used for manure; some have edible root-stocks. “Pondweed.” (Plate 4.) =Potamogeton= Tourn. 5. Perianth none. Stamens 2. Pollen-grains filiform. Carpels 2. Stigmas strap-shaped, longer than the style. Embryo with an accumbent cotyledon. Marine plants. [Tribe CYMODOCEAE.] 6 Perianth present, at least in the female flowers. Stamens 1-2. Pollen-grains globular. Carpels 3-9. Stigma shield-or funnel-shaped, shorter than the style. Embryo with a hooked or rolled cotyledon. Fresh-or brackish-water plants. [Tribe ZANICHELLIEAE.] 7 6. Stigma 1. Anthers inserted at slightly different heights. Ripe carpels scarcely compressed.--Species 2. Indian Ocean, Red Sea, Angola. (_Halodule_ Endl., under _Cymodocea_ Koen.) =Diplanthera= Thouars Stigmas 2. Anthers inserted at the same height. Ripe carpels compressed and keeled.--Species 5. North Africa, Senegambia, East Africa, Madagascar and neighbouring islands. (Including _Phycagrostis_ Ascherson) =Cymodocea= Koen.