Potamogeton Potamogetonaceae 75 



11. POTAMOGETONACAE Engl. Pond weed Family 



Flowers perfect, borne in spikes; anthers 4; leaves alternate, or the upper ones 

 sometimes opposite 58. Potamogeton, p. 75. 



Flowers unisexual, axillary; stamens 1 (2) ; leaves mostly opposite, filiform, 1-nerved, 

 entire 62. Zannichellia, p. 84. 



58. POTAMOGETON [Tourn.] L. Pondweed 



[Morong. The Naiadaceae of North America. Mem. Torrey Bot. Club 

 3: 11-65. 36 pi. 1893; Fryer and Bennett. Potamogetons of the British 

 Isles. 1-94. 60 col. pi. 1915; Hagstrom. Critical Researches on the Potamo- 

 getons. 1-281. 119 fig. Stockholm 1916; Fernald. The Linear-leaved North 

 American Species of Potamogeton, Section Axillares. Mem. Gray Herb. 

 3: 1-183. 40 pi. 1932.] 



Note : M. L. Fernald named and cited many of my specimens during the 

 writing of his monograph, and these specimens have greatly aided me in 

 the study of this difficult genus. I have also made free use of his mono- 

 graph, and I wish to acknowledge this assistance. 



I have never made a special effort to collect Potamogetons, and some 

 species have probably been overlooked; some which once occurred in the 

 state have doubtless been exterminated. Drainage has destroyed the plants 

 in many places. Cottages now surround most of our lakes, and the dredg- 

 ing of all kinds of aquatic vegetation to improve bathing beaches will 

 doubtless lead to extermination of some species. Many specimens are 

 covered more or less with a deposit of lime which may obscure such 

 characters as veins in the leaves. This can be removed at least in part by 

 gently brushing with a round bristle brush (about size no. 4), or in case 

 of badly incrusted linear-leaved specimens, it can be removed by immersing 

 them in dilute hydrochloric acid. After such treament the specimens 

 should be washed and dried between blotters. 



A. Plants with both floating and submerged leaves; floating leaves more or less 

 coriaceous, usually on petioles half as long to much longer than the length of 

 the blades; submerged leaves thin, ranging from linear to ovate, or sometimes 

 reduced to a mere petiole. 

 R. Submerged leaves bladeless; floating leaves large, 17-29-nerved. 



Floating leaves mostly broadly elliptic, subcordate at the base (rarely rounded), 



21-29-nerved; fruit mostly 4-5 mm long, stramineous or greenish 



1. P. natans. 



Floating leaves mostly narrowly elliptic, narrowed at the base, usually 2-5 



times as long as wide, 17-23-nerved ; fruit usually reddish (phase of this 



species, having the blades of submerged leaves rotted off) 



2. P. americanus. 



B. Submerged leaves lanceolate to ovate or linear. 



Blades of floating leaves 31-55-nerved, rounded at the base; blades of sub- 

 merged leaves mostly 25-39-nerved ; fruit 4-5.5 mm long, usually reddish. 



3. P. amplifolins. 



Blades of floating leaves with fewer than 31 nerves. 

 C. Submerged leaves linear, 0.2-13 mm wide. 



Peduncles of spikes from the axils of submerged leaves, mostly less than 1.5 

 cm long; fruit up to 1.5 mm long. 

 Blades of submerged leaves with bristle tips, 0.2-0.6 mm wide. 



