74 MISC. PUBLICATION 4 2 3, U. S. DEPT. OF AGRICULTURE 



5. Blades linear or capillary (6) . 



6. Fruiting spikes commonly 1.5 cm. long or longer, conspicuously inter- 

 rupted; flowers numerous, seldom fewer than 10 in a spike; stipules 

 adnate to the sheathing base of the leaf; leaves less than 2 mm. wide, 

 mostly 1 -nerved (7). 

 7. Leaves capillary; stigma capitate, borne on a distinct style; nutlets 

 2.5 mm. long 5. P. pectinatus. 



7. Leaves narrowly linear; stigma broad and depressed, sessile or nearly 



so; nutlets not more than 2 mm. long___ 6. P. interior. 



6. Fruiting spikes less than 1.5 cm. long; flowers commonly fewer than 10 in 

 a spike; stipules free or nearly so (8). 



8. Nutlets flat, cochleate, prominently crested; leaves capillary or nearly so, 



1 -nerved; spikes globose 2. P. diversifolius. 



8. Nutlets plump, not cochleate; leaves linear, 3- to 5-nerved, the lateral 

 nerves sometimes obscure (9) . 

 9. Leaves not glandular at base; peduncles commonly less than 1 cm. 

 long; spikes continuous, short-ovoid or globose; nutlets rounded- 

 lenticular, conspicuously keeled or crested on the back; winter 



buds none 7. P. foliosus. 



9. Leaves biglandular at base; peduncles up to 3 cm. long; spikes often 

 interrupted, commonly noticeably longer than wide; nutlets 

 obliquely ellipsoid, not conspicuously keeled or crested; winter 

 buds sometimes present 8. P. berchtoldt. 



1. Potamogeton natans L., Sp. PL 126. 1753. 



San Francisco Peaks, Coconino County (Thornber in 1907), Mesa, 

 Maricopa County (Tourney 497), Marsh Lake, White Mountains, 

 Apache County (Goldman 2453), 1,200 to at least 7,000 feet. Widely 

 distributed in the Northern Hemisphere. 



The last-mentioned collection is of doubtful identity, the blades of 

 the floating leaves being exceptionally narrow for the species and 

 not cordate. 



2. Potamogeton diversifolius Raf., Med. Repos. N. Y., ser. 2, 5: 354. 



180S. 



Potamogeton hybridus Michx., Fl. Bor. Amer. 1: 101. 1803. 

 Not ThuilL, 1790. 



Grandview Trail, Grand Canyon, Coconino County (Thornber 

 8506). Maine to Florida and westward. 



3. Potamogeton gramineus L., Sp. PL 127. 1753. 



Northern and central Arizona (Navajo, Coconino, Yavapai, and 

 Gila Counties). Widely distributed in North America; also in 

 Eurasia. 



The form occurring in Arizona is var. gramintfolius Fries (P. 

 heterophyllus of authors, not of Schreb.). 



4. Potamogeton zizii Roth, Enum. PL Phaen. Germ. 1: 531. 1827. 



Potamogeton angustifolius Bercht. and Presl, Rost. 1:19. 1823. 

 Not DC, 1805. 



Montezuma Well, Yavapai County (Taylor 78, Jackson 52), alti- 

 tude 3,500 feet. Widely distributed in North America; Eurasia. 



The immature specimens cited above are referred doubtfully to this 

 species. 



5. Potamogeton pectinatus L., Sp. PL 127. 1753. 



Near Tucson, Pima County (Thornber in 1913). Almost throughout 

 North America; cosmopolitan. 



