36 TETRANDRIA TETRAGYNIA. [CI. 4. 



4. P. densum, 6. 397. (Close-leaved P.) Leaves ovate, pointed, 



opposite, r rowded. Stem forked. Spike of four flowers. 

 Pools and sl«»«r riv. If. 6. Smaller, immersed. Spike short, emersed. 



5. P. rufescens, P. fluitans 18. 1286. (Reddish P.) Lower leaves 



lanceolate, acute, membranous, with distinct ribs ; upper elliptic- 

 oblong - , coriaceous, stalked, obtuse, floating'. 

 Ditches and rivul. If. 8, 9. Leaf ribs distinct. Of a reddish hue. 



6. P. lucens, 6. 376. (Shining P.) Leaves elliptic-lanceolate, pointed, 



membranous, stalked, repeatedly triple-ribbed, all submersed. 

 Spike dense, many-flowered. 

 Ponds & lakes. If. 6, 7. Larger, olive-green, or reddish. Spike green. 

 *6. P. prcelongum. (Long-stalked P.) Leaves oblong, obtuse, with 

 three principal and several lesser parallel ribs, arising from the 

 base, connected by reticulations. Peduncles elongated. Spikes 

 cylindrical, many-flowered. 

 Pools, Scotland. If . 7. Nearly as large as P. lucens. Leaves 

 semiamplexicaid. Peduncles very long. 



7. P. lanceoldtum, 28. 1985. (Lanceolate P.) Leaves lanceolate, 



membranous, flat, entire ; contracted at the base ; with chain-like 

 reticulations near the ribs. Spikes ovate, dense, of few flowers. 

 Anglesea and Scot. If. 8. Stip. narrow. Fl. brownish. 



8. P. crispum, 15. 1012. (Curled P.) Leaves lanceolate, waved, 



serrated, alternate ; the upper ones opposite. Flowers in loose 

 spikes. 

 Ditch, and rivul. If. 6, 7. Wholly under ivater. Fl. yellowish green. 



9. P. compression, 6. 418. (Flat-stalked P.) Leaves linear, obtuse, 



with a very slight point ; two lateral ribs meeting just below the 

 extremity. Stem compressed. 



Ditches and slow streams. If.. 6, 1. Stip. sheathing. Fl. brownish, 

 about 4 in a spike- 

 *9. P. acutifolium, Suppl. 2609. (Sharp-leaved P.) Leaves linear, 

 acuminate, with three principal ribs, and numerous close parallel 

 intermediate ones, occupying the whole surface. Spikes oval, 

 compact, about equal in length with the short peduncle. 



Ditches, Sussex, rare. If. 7. 



10. P. zostercefolium, Suppl. 2685. (Pointed-leaved P.) Leaves 

 broadly linear, acute, with three principal ribs, and numerous 

 intermediate ones, all distinct. Spikes cylindrical, upon long 

 peduncles. 



Ponds and rivul. If. 7. Larger, bright green. Fl.-stalks very long. 

 P. cuspidatum. E. FL 



11. P. gramineum, 32. 2253. (Grassy P.) Leaves linear, tapering 

 downward, with solitary, very slender, lateral ribs. Stem round, 

 forked. Flower-stalks from the forks, scarcely longer than the spikes. 



Ponds and ditches. If. 7. Submersed. Stipulas convoluted. 



12. P. pusillum, 3. 215. (Small P.) Leaves linear, spreading at the 

 base, opposite or alternate, with solitary, very slender, lateral 

 ribs. Stem round. Flower-stalks axillary, mostly lateral, many 

 times longer than their spikes. 



