76 Potamogetonaceae Potamogeton 



Leaves thin, distinctly several-nerved; fruit with a sharp ridge on the 



back, the sides concave 4. P. capillaceus. 



Leaves 1-nerved (under high magnification 3-nerved) ; fruit rounded on 

 the back, the sides convex. (See excluded species no. 32, p. 1024.) 



P. Vaseyi. 



Blades of submerged leaves rounded, subobtuse or acute at the tips, 



0.5-2 mm wide. 



Submerged leaves obtuse, usually rounded at the tip; the connate leaf 



sheath much longer than the free stipular tip; the space between the 



midrib and the faint lateral nerves usually filled with lacunae; fruit 



1.3-2.2 mm long, mostly about 2 mm long. (Should be sought in 



northern Indiana.) P- Spirillus. 



Submerged leaves subobtuse to acute; the connate leaf sheath about half 

 the length of the free stipular tip; the midrib of leaves rarely with 



lacunae; fruit 1-1.5 mm long 5. P. diver si folius. 



Peduncles from the axils of submerged leaves, more than 1.5 cm long. 

 Floating leaves obtuse at the apex; submerged leaves 6-14 cm long and up 

 to 10 mm wide, ribbonlike, the sides nearly parallel ... 14. P. epihydrus. 

 Floating leaves acute at the apex; submerged leaves 1-3 cm long, 2-13 mm 



wide, apiculate, broadest about the middle 



6. P. gramineus var. graminifolius. 



C. Submerged leaves lanceolate to ovate; floating leaves large. 



Floating leaves broad and distinctly cordate at the base, 25-37-nerved ; sub- 

 merged leaves 9-19-nerved; fruit 3-4 mm long 7. P. pulcher. 



Floating leaves mostly broadly or narrowly elliptic, rounded or narrowed at 



the base, generally 17-27-nerved; submerged leaves 7-29-nerved; fruit 



3-4 mm long. 



Blades of floating leaves usually narrowly elliptic, narrowed at the base; 



submerged leaves not recurved. 



Submerged leaves usually strongly mucronate; mature fruiting spikes 



mostly 7-8.5 mm wide ; fruit green 8. P. angustifolius. 



Submerged leaves acute or acuminate, mostly 17-23-nerved; mature fruit- 

 ing spikes generally 9-11 mm wide; fruit usually tinged red 



2. P. americanus. 



Blades of floating leaves rounded at the base, 17-27-nerved; submerged 

 leaves long, usually recurved, 13-29-nerved 9. P. ilhnoensis. 



A. Plants with all the leaves submerged. 



1). Blades of leaves lanceolate, oblong or broader, not linear. 

 Leaves sessile or short-petiolate, not clasping. 



Margins of blades finely and sharply serrulate 10. P. crispus. 



Margins of blades entire or some with a few minute teeth. 

 Submerged leaves all mucronate, or long-acuminate. 



Fruit 2-2.5 mm long; submerged leaves 2.5-8 cm long 



6. P. gramineus var. graminifolius. 



Fruit 3-4 mm long; submerged leaves 4-20 cm long. 



Fruit distinctly 3-keeled 8. P. angustifolius. 



Fruit with rounded, scarcely keeled sides 11. P. lucens. 



Submerged leaves all large, not mucronate 3. P. amplifolius. 



Leaves with blades clasping the stem for half or more of its diameter. 



Blades slightly clasping, lanceolate, rounded and cucullate at the apex (in 

 dried specimens often bifid), mostly 10-30 cm long; fruit 4-5 mm long, the 

 middle dorsal rib prominent and sharply keeled; stipules large, usually not 



shredded 12. P. praelongus. 



Blades strongly clasping, lanceolate to ovate-orbicular, 1-8 (11) cm long, obtuse 

 or acute; fruit 2.5-4 mm long, the dorsal ribs inconspicuous and rounded; 

 stipules short and mostly shredded 13. P. Richardsonii. 



