370 NAIADE+E. [POTAMOGBtOH. 



alternate or distichous, rarely opposite, sheathing at tlie hasp, stipules 

 or sheathing and inserted within the petiole sheath. Flowers incon- 

 spicuous, 1-2-sexual, green. Perianth 0, or cup-shaped, or of 3-4 inferior 

 valvate green sepals. Stamens hypogynous ; anthers 1-2-celled. Ovnry 

 of 1-4 carpels ; style 1, stigma various ; ovules 1 in each carpel, rarely 

 more, erect or pendulous. Fruit of one or more utricles achenes or druprs. 

 Sexl solitary, testa membranous, albumen ; embryo straight or curved, 

 radicle very large. DISTRIB. All climates ; genera about 20 ; spi-i-ics 

 100 ? AFFINITIES. With Aliamaceee. PKOPEKTIES. Unimportant. 

 TRIBE I. POTAME'.ffl. Spathe 0. Flown in spikes or clusters, solitary, 

 1-2 sexual. Stigma subcapitate or shortly decurrent. Embryo curved. 



Flowers 2-sexual ; sepals and stamens 4 each 1. Potamogeton. 



Flowers 2-sexual ; perianth ; stamens 4 2. Ruppia. 



Flowers 1 -sexual ; perianth cup-shaped ; stamen 1 3. Zannichellm. 



TRIBE II. NA lADEJE proper. Flows solitary, axillary, or on a spadix, 

 1-sexual, spathe 2-valved. Stigmas 1-3, slender. Embryo straight. 



Flowers sessile on a flat linear spadix 4. Zostera. 



Flowers axillary 5. Naias. 



1. POTAMOGE'TON, L. 



Aquatic herbs. Leaves submerged and translucent, or floating and opaque, 

 j-lternate or opposite ; stipules connate, membranous or 0. F/""-< r* 

 2-sexual, in axillary or terminal spikes, spathe basal simple. Pf.n ' until - 

 segments 4, small, herbaceous, clawed, persistent, valvate in bud. 

 Stamens 4 ; anthers subsessile on the claw of the segments, 2-rclled, 

 dehiscence outwards. Ovary of 4 (rarely 1) free sessile carpels, sti^nm 

 subsessile ; ovule 1 in each carpel, ascending, campylotropous. />>//, 4 

 (rarely 1), small, green. Seed curved round a lateral process from the cell, 

 testa membranous ; embryo hooked or annular, radicular end large point- 

 ing downwards, plumule enclosed in the cotyledons. DISTRIB. Temp, 

 regions, more rare in trop. ; species about 50. ETYM. worn/ids, a river, and 

 yflrtaf, a mdgkbour. 



SECTION 1. Leaves alternate, or the upper opposite, oblong obovate or 

 lanceolate, not truly linear, margins involute in bud ; stipules free. 

 * Flavoring item irithovt barren branches below ; upper leaves oblong flouting, 



lower (rarely all) submerged ; peduncles axillary many- and dfiau-jhun n </. 

 1. P. natans, L. ; submerged leaves or linear or lanceolate, flouting 

 broader many-nerved, petiole usually slender, peduncle not enlarged up- 

 wards, stipules large not winged, dry dnipe obovoid. 



Ponds, lakes, and ditches ; fl. July-Sept. Stem terete. Floating leaves 2-6 in., 

 oblong-ovate obovate or lanceolate, obtxise or acute at the base, olive-green, 

 submerged paler ; stipules 1-4 in. Peduncles very variable in stoutm- s ,md 

 length. Drupe (dry) with the ventral margin convex, dorsal ^-circular, keeled 

 below ; beak short. DISTRIB. Almost ubiquitous. 



Sub-sp. NA'TANS proper; floating leaves coriaceoxis, blade decurrent along 

 the petiole, submerged linear coriaceous, stipules large, peduncles stout, 

 flowers large dense, dried drupe in. green. Common at low elevations. 

 Sub-sp. POLYGONIKO'LICS, Pourrtt (sp.); leaves smaller, lower submerged linear 



