36 26, Pofcamogit. 6. FLUVIAT. Pl.end.ph. 
10. Potamogiton pectinatum. Featherleafed riverweed . 
Leaves bristlelike, alternate, the two upper ones opposite, 
parallel, close together, sheathing at the base ; spike pe- 
duncled, thin, often interrupted. 
Pot. millefolium, seu foliis gramineis, ratnosum, Raii Syn. 150, 14. 
Potamogeton pectinatum, Lin. S. P. 183. 
Fennel-leaf pondweed. 
Rivers and ponds ; perennial ; July. 
Root creeping, originally tuberous ; stem sunk, flowering 
in still water only, long, filiform, branchy, jointed, whitish ; 
sheath prolonged at top into a small membranaceous li- 
gula. 
11. Potamogiton marinum. Sea riverweed. 
Leaves linear, alternate, 2-rowed, sheathing ; margin of 
the sheath scariose ; spike fewflowered, very much inter- 
r up ted. 
Pot. maritimum ramosissimum grandiusculis capitulis, capillaceo folio-, 
Raii Syn . 150, 13. 
Potamogeton marinum, Lin. S. P. 183. 
Salt water ditches; perennial; July. 
Stem firm, reddish ; joints short ; sheath white ; leaves 
opake and shining. 
12. Potamogiton Vaillantii . Vaillanfs riverweed . 
Leaves linear, alternate, 2-rowed below, sheathing, above 
opposite, subspathaceous ; spike ovate, fewflowered ; flower 
rather large. 
P. ingens gramineo folio longiore, Vaill. Paris. 32. 
P. marinum, Teesdale Lin. Tr. 5,43. 
P.Vaillantii, Roemer and Schultze Syst. Veg. 
Stem long, naked below', branchy above, compressed, 
deeply streaked, yellowish ; lower leaves 4> in. long, very 
narrow 7 ; upper leaves 2 or 3 in. long; both light sea-green; 
spike 6 or 8-flowered, crowded together. 
13. Potamogiton gramineum. Grass riverweed. 
Leaves linear, lanceolate, alternate, sessile ; stipules nar- 
rower than the leaves ; stem round, rather forked. 
P. gramineum latiusculum, foliis et ramiticationibus densissime stipatis,, 
Raii Syn. 149, 10. 
Potamogeton gramineum, Lin. S. P. 184. 
Ditches and rivers ; perennial; July. 
Plant sunk ; stem slender, bent ; leaves close ; stipules 
involuto-turbinate ; spikes ovate, fewflowered, close, from 
the forks of the stem ; peduncles as long as the spike. 
