A GUIDE AND KEY TO AQUATIC PLANTS 89 



3. Septa becoming much closer together near top of stem ; 1. E. inter stincta. 



perianth-bristles longer than achene. 

 3. Septa evenly spaced ; perianth-bristles shorter than the 2. E. equisetoides. 

 achene. 



4. Stem definitely 3 or 4-angled 5. 



4. Stem round or sometimes flattened 6. 



5. Stem 4- \ngled, stout, mostly over 2 mm. in diameter 3. E. quadrangulata. 



5. Stem 3-angled, about 1 mm. in diameter 4. E. robbinsii. 



6. Stem stout, round, over 2 mm. in diameter 5. E. cellulosa. 



6. Stem round or sometimes flattened, about 1 6. E. elongata. 

 mm. in diameter. 



7. Achenes lenticular; 2-sided 8. 



7. Achenes 3-angled __ 12. 



8. Tubercle of achene lamellif orm (plate-like) 7. E. obtusa. 



8. Tubercle of achene not lamelliform 9. 



9. Sheaths at base of stem firm at the apex 10. 



9. Sheaths at base of stem membraneous at apex 13. 



10. Plants perennial from elongated rootstocks 11. 



10. Plants tufted and annual, without conspicu- 12. 

 ous rootstocks. 

 11. Tips of spikelet scales spreading; 2 or 3 non-fertile 8. E. smallii. 



scales at base of spikelet. 

 1 1 . Tips of spikelet scales erect ; 1 non-fertile scale present 9. E. calva. 

 at base of spikelet and this entirely encircles the 

 base. 



12. Achene 1 mm. long 10. E. geniculata. 



12. Achene 0.5 mm. long 11. E.atropurpurea. 



13. Mature achene olive colored 12. E. olivacea. 



13. Mature achene black or deep-brown 13. E.flavescens. 



14. Achene cancellate ; that is the surf ace formed 15. 



into small cells. 

 14. Achene smooth, papillose, or irregularly 18. 

 ridged. 



15. Achene with longitudinal ribs 14. E. acicularis. 



15. Achene without longitudinal ribs 16. 



16. Achene 1 mm. or less long; stems often 15. E. vivipara. 



proliferous. 

 16. Achene at least 1.5 mm. long; stems not 17. 

 proliferous. 



17. Tubercle as large as the achene or larger, mitre-like 1&. E. tuberculosa. 



17. Tubercle smaller than achene, conic 17. E. tortilis. 



18. Achenes shining, black, topped by a de- 18. E. melanocarpa. 

 pressed whitish tubercle. 



18. Achenes not black 19. 



19. Achenes with the 3 angles ribbed 19. E. tricostata. 



19. Achenes with the angles not ribbed 20. 



20. Spikelets obviously distichous; that is, scales 20. E. baldwinii. 



of spikelets in 2-ranks. 

 20. Spikelets scales not 2-ranked 21. E.microcarpa. 



1. E. interstincta (Vahl) R. & S. Spikerush — Tropical species 

 known from two Florida counties but probably occurs more widely in 

 that State. The senior writer collected it from marshes near Lake 

 Okeechobee in Glades County. 



