POND-WEED FAMILY 517 



oblong, wavy, toothed, 3 nerved, spreading, and recurved ; stipules 

 small, blunt, falling off; flowers 6 — 8 in a loose spike on a taper- 

 ing stalk ; carpels with a long beak. — Ditches ; common. — Fl. 

 June — August. Perennial. 



** Leaves nearly opposite, involute, exstipulate 



18. P. densus (Opposite-leaved Pond- weed). — Stem slender, 

 brittle, dichotomously 

 branched ; leaves all sub- 

 mersed, embracing the 

 stem, ovate, toothed, 

 pellucid, 3 — 5-nerved, 

 close - set, recurved ; 

 flowers few, in very short- 

 stalked spikes ; carpels 

 with short, recurved 

 beaks. — Ditches and 

 ponds ; common. — Fl. 

 June — September. Pe- 

 rennial. 



*** Leaves all similar, 

 strap-shaped, with flat 

 margins and free 

 stipules 



19. P. compressus. — 

 Stem compressed, wing- 

 ed; leaves 4 — 10 in., 

 acute, 3 — 5-nerved ; sti- 

 pules large, acute ; flowers 

 many, in cylindric, long- 

 stalked spikes ; carpels 

 compressed, 3-ribbed. — 



Ditches ; local. Fl. July, POTAMOG g TON D £ NSUS {flpposite-leaved Pond-weed). 



August. Perennial. 



20. P. acutifolius. — Stem compressed, narrower than in the 

 preceding ; leaves much as in the preceding but shorter and with 

 narrower stipules; flowers in a very short oval, short-stalked 

 spike ; carpels compressed, with a recurved beak. — Lakes and 

 ditches ; rare. — Fl. June — August. Perennial. 



21. P. obtusifolius (Grassy Pond-weed). — Stem slender, 4-angled, 

 much branched ; leaves sessile, linear, obtuse, dark green, opaque, 

 3-nerved ; stipules broad, blunt ; flowers few, in a dense, oval, 



