264 FLU VI ALES. [Potamogeton. 



Ord. 75. FLUVIALES. Vent. Pond-weed Family. 



Flowers separated, monoecious or dioecious, sometimes per- 

 fect. Perianth of two or four pieces, often deciduous, rarely 

 wanting. Stamens definite, hypogynous. Ovary one or more, 

 superior; stigma simple; ovule solitary, pendulous. Fruit dry, 

 not opening, 1-celled, 1-seeded. Seed pendulous; albumen 

 none ; embryo having a direction contrary to that of the seed, 

 with a lateral cleft for the emission of the plumule. — Water- 

 plants. Leaves very vascular, with parallel veins. Flowers in- 

 conspicuous, usually arranged in terminal spikes. 



1. Potamogeton. Linn. Pond-weed. 



Flowers sessile upon a spike or spadix, which issues from a 

 sheathing bractea or spatha. Perianth single, of four scales. 

 Anthers sessile, opposite the scales of the perianth. Pistils 

 four : which become four small ?iuts. Embryo curved. — 

 Name, from 77-oTa/ios, a river t and r/enwv, a neighbour. All 

 the species grow in the water, and often present as beautiful 

 an appearance in clear streams and ponds, as the Fuci do in 

 the ocean. They protect the spawn of fish, and harbour in- 

 numerable aquatic insects, affording food by their roots and 

 seeds to aquatic birds. — Chamisso and Schlechtendal have 

 well illustrated this Genus. See Linnaa, v. ii. p. 159. 



Tetrandria. Tetragynia. 



H« Leaves all opposite : stipides none. 



1. P. densus, Linn. Opposite-leaved Pond-weed. Leaves 

 all opposite, amplexicaul, ovate, acuminate or lanceolate. Br. 

 Fl. 1. p. 73. E. Fl. v. i. p. 230. E. Bot. t. 397. 



Ditches. About Limerick and elsewhere, not unfrequent ; Mr. W. 

 H.Harvey. Fl. June, July. %. — Head of Jlowers small, rounded. 

 Leaves keeled below, middle nerve or rib of many longitudinal cells, 

 with two, and sometimes three lateral parallel veins on each side, the 

 inner one the strongest. 



* * Leaves alternate, all submersed, with adnata stipules. 



2. P. pectinatus, Linn. Fennel-leaved Pond-iveed. Leaves 

 distichous, setaceous, single-nerved ; sheathing by means of 

 their adnate stipules ; spike interrupted. Br. Fl. 1. p. 73. E. 

 Fl. v. i. p. 236. E. Bot. t. 323. 



Rivers, lakes, and salt-water ditches, frequent. FL July. %. — 

 Root tuberous, with creeping scions. Stems very much branched, 

 various in length, leafy, zigzag. Spikes few, solitary, each from one of 

 the uppermost forks of the branches, on a longish stalk, cylindrical, 



