Jones: Flora of Illinois, 18. Potamogetonaceae 43 



3. Stipules adnate to the base of the leaves. 



7. Leaves filiform, less than 0.6 mm. wide, entire; lakes, rare 



P. pectinatus L. 



7. Leaves linear, 2-ranked, 3-6 mm. wide, the margin microscopically 



serrulate; lakes, rare P. robbinsii Oakes 



2. Leaves lanceolate to oval or ovate, L5-6 cm. wide. 

 8. Leaves perfoliate. 



9. Leaves slightly clasping at base, cucullate at the apex, mostly 8-30 



cm. long; fruits 4-5 mm. long; lakes, n.e. Ill 



P. praelongus Wulfen 



9. Leaves mostly 2-8 cm. long, strongly clasping at the base, the apex 



flat, not cucullate; fruits 2.5-4 mm. long; lakes, n.e. 111., rare 



P. richardsonii (A. Benn.) Rydb. 



8. Leaves not perfoliate. 



10. Leaves oval or lanceolate, the margin often crisped; fruit merely 



apiculate; lakes, local P. lucens L. 



10. Leaves linear-oblanceolate, serrulate throughout; beak of fruit 2-3 

 mm. long; ponds and streams, not common; known from Cook, 



Tazewell, and Vermilion counti'es; native of Eur P. aispus L. 



1 . Leaves of two kinds, broader floating ones, and narrower submerged ones. 



11. Submerged leaves lanceolate to elliptical, more than 5 mm. wide. 



12. Stem usually black-spotted; principal floating leaves somewhat 

 cordate at base; fruit 3-3.5 mm. long; shallow water, n.e. 111., 

 rare P. pulcher Tuckerm. 



12. Stem not black-spotted; leaves tapering at the base, or rounded. 



13. Floating leaves with 30 or more principal veins; lakes and 



ditches, local P. amplifolius Tuckerm. 



13. Floating leaves with fewer veins. 

 14. Mature spikes 4-6 cm. long. 



15. Submerged leaves sessile, the apex mucronate; fruit dis- 

 tinctly 3-keeled; lakes, local 



P. angustifolius Bercht. dC Presl 



15. Submerged leaves petioled, the apex acuminate. 



16. Floating leaves elliptical, not mucronate, 4-9 cm. 

 broad; submerged leaves lanceolate; style prominent 

 on the fruit; streams and ponds in the northern half 

 of the state; first collected near Oquawka, Hender- 

 son Co., by H. N. Patterson 



P. illinoensis Morong 



16. Floating leaves oval, mucronate, 1-3 cm. wide; sub- 

 merged leaves narrowly lanceolate; fruit tipped by 

 the nearly sessile stigma; ponds, ditches, and streams, 



not uncommon [P. nodosus Poir. (?)] -.- 



P. americanus C. & S. 



14. Mature spikes 1-2 cm. long; floating leaves oval, 1-3 cm. 

 wide; submerged leaves lanceolate, acuminate or cuspidate; 



