245 



mm. long and i ^ to i^ mm. wide. Carpels two-keeled and 

 usually a little scabrate on the back. 



Ontario, New York, south to Florida, New Mexico and Mex- 

 ico, west to Ohio and Minnesota. 



8. M. hippiiroides. Nutt. in Torrey and Gray Fl. i, 530(1840). 



M. scabratiivt. Cham. Linnaea 4, 506, not Michaux. 



Leaves in whorls of fours and sixes, sometimes scattered, the 

 floral 8 to 12 mm. long, linear-lanceolate, serrate or dentate, up- 

 permost nearly or quite entire, lowest pinnatifid ; submerged, with 

 six to eight pairs of capillary pinnae. Monoecious, but occasion- 

 ally with only one kind of flowers on the stems, and so appearing 

 dioecious. Petals often pink-colored, sub-persistent. Stamens 

 four. Fruit about 2 mm. long by i mm. or a little more in 

 breadth ; carpels keeled and somewhat rough, with a deep groove 

 between them. A western form nearly allied to our Atlantic M. 

 heterophyllmn. Coll. by Nuttall and Hall in the Wahlamet ponds, 

 and by Howell at Sauvie's Island, Oregon, and by Chamisso near 

 San Francisco, E. L. Greene, Stockton, and A. B. Simonds at 

 Ckar Lake, California. 



9. M. pifinatutn. (Walt.) 



Potamogeton pinnaUtm. Walt. Fl. Car. 90 (1788). 



M. scabratum. Michx. Fl. 2, 190 (1803), and most Amer- 

 ican authors. 



Leaves in whorls of threes and fives, sometimes scattered, the 

 floral linear, pectinate, toothed or cut-serrate, the teeth compara- 

 tively few, 5 to 15 mm. long, gradually changing into the sub- 

 merged, which are in crowded verticils, the capillary pinnae sparse. 

 Spikes 10 to 20 cm. long. Petals purplish, somewhat per- 

 sistent. Stamens four, very rarely six. Mature fruit about i ^4 

 mm. long, and i^ mm. broad. Carpels strongly two- keeled and 

 scabrate on the back, the grooves deep. 



Shallow water, Rhode Island to Florida, Louisiana, Texas and 

 Missouri. 



10. M. Mexicanum. S. Watson Proc. Am. Ac. 25, 148 (1890). 

 Stems stout, much branched. Leaves in verticils, or subver- 



ticils of fours and fives, or scattered, the floral pinnatifid or toothed, 

 the divisions remote, 10 to i 5 mm. in length ; submersed pin- 

 natifid, 20 to 30 mm. long, the divisions finely capillary, 



