MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 21 
eroded, glabrous or pubescent, mostly ciliate, petioled or sessile, often glandular-punctate. 
Ocreae cylindric, becoming funnelform at branching nodes, mostly membranous, trun- 
cate, naked, ciliate or fringed with bristles, ribbed, striate, often brittle. Inflorescence 
terminal, geminate or paniculate, more or less compound or rarely simple, the ultimate 
divisions ending in spicate racemes. Racemes varying from linear to ovoid, dense and 
erect, or few-flowered, lax and drooping. Ocreolae funnelform, naked, ciliate or fringed 
with short bristles. Pedicels rather stout, articulated at the base of the calyx. Calyx 
more or less colored, varying from white and green to red, often glandular-punctate, five- 
cleft or five-parted, investing the achene by enlargement after flowering. Stamens vary- 
ing from four to eight, included or exserted. Style mostly two-cleft or two-parted, 
sometimes three-cleft or three-parted on the same plant, included or exserted, stigmas 
capitate. Achene mostly lenticular, sometimes triquetrous and lenticular on the same 
plant, usually black, smooth or granular, its faces concave or convex, its edges rounded 
or sharp. Albumen horny. Cotyledons accumbent. 
KEY TO THE SPECIES. 
Racemes usually solitary (sometimes two); plants more or less aquatic. 
Ocreae neither spreading at summit nor fringed. 
Leaves elliptic, oblong or elliptic-lanceolate, obtuse. 7. P. AMPHIBIUM. 
Leaves ovate-lanceolate or oblong-lanceolate, acuminate. 9. P. EMERSUM. 
Ocreae spreading at summit and mostly fringed. 8. P. HarrwrRiGuHrit. 
Racemes numerous (at least several), more or less paniculate or geminate ; 
plants normally terrestrial. 
Ocreae cylindric, naked (occasionally ciliate when young). 
Racemes erect; achenes only lenticular. 
Achene strongly biconvex; style longer than the achene, parted 
to below the middle. , 10. P. PoRTORICENSE. 
Achene strongly biconvex; style shorter than the achene, parted 
to near the base. 11. P. FERRUGINEUM. 
Achene flat, ovoid, gibbous on one side, thick-pointed; style 
short. 17. P. Mexicanum. 
Achene flat, ovoid, gibbous on both sides, slender-pointed ; style 
exserted. 18. P. LONGISTYLUM. 
Achene flat, orbicular, not gibbous; style mostly included. 19. P. PENNSYLVANICUM. 
Racemes erect; achenes both lenticular and triquetrous. 16. P. PRINGLEI. 
Racemes drooping ; achenes mostly lenticular, sometimes triquetrous. 
Achene broadly oblong-ovoid; flowers flesh-colored; style two- 
parted to below the middle. 14. P. LAPATHIFOLIUM. 
Achene ovoid; flowers whitish, green or rose-colored; style two- 
parted to near the base. 15. P. INCARNATUM. 
