NAIADACE^. 223 



10. P. prselon'gUS, Wulfen. Stem very long and branch- 

 ing. Leaves all submersed and similar, lanceolate, half- 

 clasping, with a boat-shaped cavity at the end. Spikes 

 loose-flowered with very long peduncles. Fruit sharply 

 keeled when dry. Stem white. — Ponds and large rivers. 



11. P. perfolia'tUS, L. Leaves all submersed, varying 

 in width from orbicular to lanceolate, clasping by a heart- 

 shaped base. Stem branching. 



Var. laneeola'tUS, Bobbins, has long-lanceolate acum- 

 inate leaves. Peduncles thickened upward. 



* * * Leaves all submersed and similar^ jnostly sessile, linear or bristle- 

 like. 



12. P. eompressus, Fries. {P. zoster <sf alius, Schum., in 

 Macoun's Catalogue.) Leaves linear, grass-like, sessile, 

 abruptly pointed, with three large nerves and many fine ones. 

 Stem branching, wing-flattened. Stipules free from the 

 sheathing base of the leaf. Spikes cylindrical. 



13. P. paueiflO'rUS, Pursh. Stem filiform, flattish and 

 much branched. Leaves narrowly-linear, acute, obscurely 

 3-nerved. Spikes capitate. 



14. P, pusil'lUS, L. Stem slender, somewhat flattish, 

 often muck branched. Leaves narrow- or setaceous-linear, 

 1— S-nerved, with translucent glands on each side at the base. 



15. P. mueronat'US, Schrad. Like the last, but the stem 

 less branching, and the leaves broader (nearly an inch wide), 

 often 5-nerved. 



16. P. peetina'tUS, L. Leaves bristle-shaped. Stem 

 repeatedly forking, filiform. Spikes interrupted, on long 

 slender pedu/ndes. Stipules united with the sheathing base of 

 the leaf. 



17. P. Robbin'sii, Oakes. Leaves narrowly lanceolate 

 or linear, crowded in 2 ranks, recurved, serrulate, many- 

 nerved. Stems rigid with numerous branches. Fruit 

 keeled with a, broadish wing. Stipules united with the 

 sheathing base of the leaf. 



