FLUVIALES. 385 



HAB, Ditches. N, S. Torn July, Aug. %j$.Stem long, fili- 

 form. Leaves opposite, linear. Flowers axillary, small. 



Horn Pond-weed. 



2. Z. intermedia Torr. : anther 2-celled ; stigmas dentate-crenate ; 

 seed smooth, entire on the back. 



HAB. Salt marsh ditches. July. 0. Submerged. Stem fili- 

 form. Leaves entire. Flowers inconspicuous. Torr. 



4. RUPPIA. Linn. 



Flowers 2, perfect, on a spadix arising from the sheathing 

 base of the leaves. Stamens 4, sessile. Ovaries 4. Peri- 

 anth none. Drupes 4, pedicellate ; nuts 1-seeded. 



Tetrandria. Teiragynia. 



R. maritima Linn. 



HAB. Salt marshes. N. J. to Geor. July. If. Stem long, 

 filiform, branched, floating. Leaves linear, setaceous, with in- 

 flated sheaths. Spadix short, with 2 naked green flowers. An- 

 thers large. 



5. POTAMOGETON. Linn. 



Flowers perfect, on a spadix arising from a spathe. Peri- 

 anth single, 4-leaved. Anthers 4, sessile, alternating with 

 the divisions of the,perianlh. Ovaries 4. Nuts 4, 1-seeded, 

 sessile. Tetrandria. Tetragynia. 



* Upper leaves floating. 



1. P. natans Linn. : upper leaves floating, coriaceous, on long peti- 

 oles, oblong-ovate ; lower ones membranous, linear-lanceolate, gradu- 

 ally tapering into a petiole. P. natans b. Mich. 



HAB. Ponds and lakes. N. S. July, Aug. 11. Stem varying 

 in length. Leaves sometimes cordate. Spadix 1 2 inches 

 long, rising above the water. Pond Weed. 



2. P. fluitans Linn. : upper leaves floating, subcoriaceous, ovate- 

 lanceolate, obtuse, tapering into a rather short petiole ; lower ones 

 very long, lanceolate, membranous and sessile. 



HAB. Ponds and streams. N. Y. to Car. July, Aug. If. 

 Stem varying in length. Leaves reddish, less coriaceous than in 

 the preceding. Spadix an inch long, almost submerged. 



3. P. heterophyllum Schreb. : upper leaves floating, coriaceous, ellip- 

 tical, petiolate ; lower ones membranous, linear-lanceolate, sessile. 

 P. hybridum Mich. 



HAB. Stagnant water. N. Y. to Car. Aug. 1. Smaller than 

 the former. In flowing water the lower leaves are very long 

 and narrow. 



4. P. dicersifolium Bart. : upper leaves floating, elliptical, petiolate, 



as 



