6 ZANNICHELLIACEAE 



Submerged leaves without blades. 



Drupelets barely keeled : nutlets pitted. i. P. natans. 



Drupelets 3-keeled : nutlets not pitted. 2. P. Oakesiamis. 



Submerged leaves with flat blades. 



Submerged leaves of 2 kinds, some with narrow blades, others with broad 

 blades. 

 Upper submerged leaves with broad blades ; lower with narrow blades. 



3. P. amplifolius. 

 Upper submerged leaves with narrow blades ; lower with broad blades. 



4. P. pulcher. 

 Submerged leaves all alike 



Blades of the submerged leaves linear or nearly so throughout. 

 Blades the same breadth throughout, coarsely cellular-reticulated in 



the middle. 5. P. Nuttallii. 



Blades dilated at the base, without cellular reticulation. 



8. P. heterophyllus. 

 Blades of the submerged leaves lanceolate. 

 'Leaves with petioled blades or the lower sometimes sessile. 

 Upper leaves with petioled blades, the lower blades sessile. 



6. P. alpinus. 

 All the leaves with petioled blades. 7. P. lonchiies. 



Leaves with sessile or subsessile blades. 9. P. Zizii. 



b. Plants with submerged leaves only. 



Plants without propagative buds and without glands. 

 Leaf-blades several-many-nerved : 



Blades relatively broad, lanceolate to ovate, many-nerved. 

 Leaf-blades sessile or nearly so, acute or cuspidate. 



10. P. lucens. 

 Leaf-blades meeting around the stem, obtuse. 11. P. per/oiiatus. 

 Blades relatively narrow, linear to oblong-linear, several-nerved. 



15. P.foliosus. 

 Leaf-blades capillary or setaceous, i -nerved or nerveless. 



12. P. confervoides. 

 Plants with either propagative buds or glands, or both. 

 Propagative buds present : glands wanting. 

 Leaf-blades 3-7-nerved, serrulate. 13. P. crispns. 



Leaf-blades with 3 main nerves and many delicate ones, entire. 



14. P. zosteraefolius. 

 Propagative buds and glands both present. 

 Glands large and translucent : buds rare. 16. P. oblusifolius. 



Glands small, often dull : buds common. 17. P. pusillus. 



B. Stipules adnate to the leaf-blade or petiole. 

 Plants with both svxbmerged and floating leaves. 

 Submerged peduncles as long as the spikes, clavate, often recurved. 



18. P. diversifolius. 

 Submerged spikes sessile or nearly so. 19. P. Spirillus. 



Plants with submerged leaves only. 

 Drupelets not keeled or only obscurely so : leaf-blades setaceous. 



20. P. pectinalus. 

 Drupelets strongly 3-keeled : leaf -blades linear. 21. P. Robbinsii. 



1. Potamogeton natans L. Common Floating Pondweed. (Man. 

 p. 42; I. F. y. 142.) In ponds and streams, nearly throughout N. A. 



