48 Karl M. Wiegand and Arthur J. Eames 



a. Ligules united with the leaf base for a considerable distance, sheathing the stem ; 

 floating leaves lacking. 

 b. Leaves 3 mm. wide or less, not auricled nor definitely 2-ranked ; entire. 

 c. Stigmas scarcely visible on the fruit, broad and sessile; leaves refuse, blunt, 

 or often shortly apiculate. 

 d. Whorls of flowers 5-12, evenly spaced or the lower ones somewhat remote ; 

 primary sheaths swollen, two to five times thicker than the stem ; primary 

 leaves short and ribbon-like, upper leaves filiform ; plants long and coarse. 



20. P. vaginatus 

 d. Whorls of flowers 2-5, the upper ones approximate, the lower ones remote ; 

 sheaths tight, leaves all filiform; plants short and fine. 21. P. filiformis, 



var. borealis 

 c. Stigma raised on a minute style, capitate, narrow ; leaves gradually acuminate. 



22. P. pectinatus 

 b. Leaves 4-8 mm. wide, auricled at the base, stiffly 2-ranked, with a cartilaginous, 

 finely and sharply serrate, margin. 23. P. Robbinsii 



1. P. natans L. 



Shallow ponds and stagnant water ; frequent. Fr. July-Sept. 



Spencer Lake ; Red Mill Pond ; Renwick ; s. w. corner of Cayuga Lake ; Duck Lake ; 

 and elsewhere. 



Newf. to B. C, southw. to n. N. J., Pa., Mo., Nebr., and Calif.; almost cosmo- 

 politan in the temperate countries. 



2. P. amplifolius Tuckerm. 



Ponds, slow streams, and lake waters ; frequent. Fr. Aug. 



Spencer Lake; Cayuta Lake (D.) ; mouth of Fall Creek (D. !) ; Freeville; Lake 

 Como (Locke Pond, D.) ; Dryden Lake (D.) ; Cayuga; Black Brook, Tyre. 



N. S. to B. C, southw. to n. N. J., Ky., Kans., and Calif. ; rare or absent on the 

 Atlantic Coastal Plain. 



3. P. americanus C. & S., var. novaeboracensis (Morong) Benn. (P. fluitans of 



Gray's Man., ed. 6. P. lonchites of Cayuga Fl. P. natans, var prolixus, of 

 Cayuga FL?) 



Slow streams and quiet lake waters ; frequent. Fr. Aug.-Sept. 



Spencer Lake; Dryden Lake (D.) ; mouth of Fall Creek (Z). !); Renwick, on 

 the lake front ; outlet to North Spring, Union Springs ; Canoga Marshes ; Cayuga 

 (D.) ; Erie and Seneca Canals (£>.!). 



N. B. to B. C.j southw. to Fla., Tex., Mex., and Calif., including the Atlantic 

 Coastal Plain. Found also in W. I. 



The submerged leaves are usually absent at fruiting time, and the plant is then 

 sometimes mistaken for P. natans. 



4. P. gramineus L., var. graminifolius Fries. (See Rhodora 23:189. 1921. 



P. heterophyllus and forma graminifolius of Gray's Man., ed. 7. P. gramineus 

 of Cayuga Fl.) 

 Ponds, slow streams, and lake borders ; frequent. Fr. Aug. 



Spencer Lake; Summit Marsh (D.l) ; Cayuta Lake (£>.); near the Ithaca light- 

 house (£>.) ; Myers Point (D. !) ; Ledyard; Union Springs; Cayuga Marshes. At 

 Spencer Lake, where the water is shallow and the bottom is sandy or marly, the 

 plant approaches P. heterophyllus, forma myriophyllus (Robbins) Morong. 



Widely spread throughout the greater part of N. A. ; common along the coast ; 

 very variable. 



