NAIADACE.E. 223 



10. P. pPSBlon'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. 



Yar. lanceola'tus, Bobbins, has long-lanceolate acum- 

 inate leaves. Peduncles thickened upward. 



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

 like. 



12. P. COmpressus, Fries. (P. zostercefolius, 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. p^usil'lus, L. Stem slender, somewhat flattish, 

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

 1-3-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 peduncles. 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. 



