A GUIDE AND KEY TO AQUATIC PLANTS 



69 



1. S. eurycarpum Engelm, Bur-reed — Not common in the south- 

 east, but noted north of our territory in association with Anopheles 

 quadrimaculatus . 



2. S. americanum Nutt. Bur-reed — Not collected by the writers 

 but supposed to occur over most of our territory. 



NAJADACEAE Pondweed Family 



1. Potamogeton [Tourn.] L. 



I. Plants with both floating and submerged leaves, the 2. 

 floating leaves with expanded blades and differing 

 from submerged ones. 



1. Plants with all leaves alike and submerged 10. 



2. Submerged leaves filiform or very narrowly 3. 



linear, less than 2 mm. wide. 

 2. Submerged leaves lanceolate or ovate or if 5. 

 linear more than 2 mm. wide. 



3. Flowers in cylindrical, many-flowered spikes only 1. P. nutans. 



3. Flowers in globular, few-flowered spikes in the axils of 4. 

 the submerged leaves as well as in cylindrical, 

 many-flowered emersed spikes. 



4. Submerged leaves 0.5-2 mm. wide, not taper- 2. P. diver sijoliua. 

 ing to a long point; floating leaves, when 

 present, rounded at the tip, the larger 5-15 

 nerved. 

 4. Submerged leaves 0.1-0.6 mm. wide, taper- 3. P. capillaceus. 

 ing to a long point; floating leaves, when 

 present pointed at the tip, 3-7 nerved 



5. Submerged leaves linear and ribbon-like 4. P. epihydrus. 



5. Submerged leaves lanceolate or ovate 6. 



6. Floating leaves heart-shaped at base 5. P. pulcher. 



6. Floating leaves rounded or tapering at base. 7. 



7. Floating leaves with 30-50 nerves 6. P. amplifoUus. 



7. Floating leaves with less than 30 nerves 8. 



8. Upper submerged leaves with long petioles. _ 7. P. nodosus. 

 8. Upper submerged leaves with obscure short 9. 

 petioles or sessile. 

 9. Submerged leaves with an abrupt awl-shaped tip, 2-4 8. P. angustifoUua 

 mm. long. 



9. Submerged leaves with no awl-shaped tip. 9. P. gramineus. 



10. Leaves lanceolate or broader, with curved 11. 



not parallel sides. 

 10. Leaves thread-like or ribbon-like, linear 15. 

 with parallel sides. 

 11. Leaves short petioled or sessile but not clasping at 12. 

 base. 



11. Leaves clasping at base 14. 



12. Leaves with an awl-shaped tip 2-4 mm. long 13. 



12. Leaves without awl-shaped tip 9. P. gramineus. 



13. Fruit with a single keel 10. P. illinoensis. 



13. Fruit with 3 distinct keels 8. P. angustifolius. 



