"74 ^- POTAMOGETOXACEAE 



3. Spikes compound, submerged Spikelets shorter than their bracts. Flowers 



polygamous. Stamens 3, rarely 4. Pollen-grains fiUform. Carpel i. 

 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 i. 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 POTAMOGE- 

 TONEAE.] 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 i 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 CYMODOCEEAE.] .... 6 



Perianth present, at least in the female flowers. Stamens i — 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 i. Anthers inserted at shghtly different heights. Ripe carpels 

 scarcely compressed. — Species 2. Indian Ocean, Red Sea, Angola. 

 [Halodule Endl., under Cymodocea Koen.) Diplanthera Thouars 



Stigmas 2. Anthers inserted a,t the same height. Ripe carpels compressed 

 and keeled. — ■ Species 5. North Africa, Senegambia, East Africa, 

 Madagascar and neighbouring islands. (Including Phycagrostis Ascher- 

 son) Cymodocea Koen. 



7. Perianth in the male flowers none, in the female cup-shaped and imdivided. 

 Anthers stalked, opening by 2 longitudinal slits. Carpels usually 4, 

 slightly curved, with a peltate stigma. — Species i. North and South 

 Africa, southern West Africa, Madagascar and neighbouring islands. 



Zannichellia Mich. 



Perianth in the male flowers 3-toothed, in the female consisting of i — 3 



segments. Anthers sessile, opening with one longitudinal slit. Carpels 



3, straight, with a funnel-shaped stigma. — Species i. North-west 



Africa (Algeria) Althenia Fr. Petit 



