22 ZANNICHELLIACEAE 



Perianth none, but flowers sometimes in hyaline envelopes. Stamens 1-4, 

 seldom more, distinct and hypogynous in the perfect flowers, or solitary 

 in the staminate ones. Ovaries 1-4, distinct, 1-celled and 1-ovuled. Fruit 

 mostty drupelets or achenes. 



Stamens 4, the connectives with dilated appendages ; drupelets sessile. 1. POTAMOGETON. 

 Stamens 1-2, the connectives without appendages; drupelets manifestly stipitate. 



Stigmas sessile; anthers 2; flowers perfect, on long peduncles. 2. RUPPIA. 



Stigma terminating a long style; anther 1; flowers monoecious, the two kinds together 

 in the same axils. 3. ZANNICHELLIA. 



1. POTAMOGETON (Tourn.) L. PONDWEED, FISHWEED. 



Immersed water plants, with flat, 2-ranked leaves, alternate or the upper 

 opposite, often of two kinds, floating and submerged, the former more or less 

 coriaceous and broad, the latter pellucid, thin, and narrower. Stipules present, 

 free or adnate to the lower part of the petiole or blade, enclosing the young 

 flower-buds. Inflorescence spicate, axillary, mostly emersed. Stamens 4; ap- 

 pendages short-clawed, valvate in the bud. Ovaries 4, sessile, distinct, with a 

 short style or sessile stigma. Fruit of 4 drupelets. Seeds crustaceous. 



Species with both floating and submerged leaves. 



Submerged leaves bladeless. 1. P. natans. 



Submerged leaves with proper blades. 



Submerged leaves of two kinds, oval or oblong ones and lanceolate and strongly 



curved ones ; floating leaves with 30 or more nerves. 2. P. amplifolius. 



Submerged leaves of only one kind; floating leaves with less numerous nerves. 

 Stipules free from the petioles and blades. 

 Submerged leaves lanceolate. 



Submerged leaves all petipled, more than 7-nerved. 3. P. americanus. 

 Submerged leaves all sessile or the uppermost short-petioled. 



Peduncles of the same thickness as the stem; leaves not serrulate at 



the apex. 



Plant green; submerged leaves narrower than the floating ones. 



7. P. heterophyllus. 

 Plant red; submerged leaves as wide as the floating ones or 



wider. 4. P. alpinus. 



Peduncles thicker than the stem; leaves serrulate at the apex. 



5. P. augustifolius. 

 Submerged leaves linear. 



Submerged leaves of nearly the same width throughout, coarsely reticu- 

 late in the middle. 6. P. epihydrus. 

 Submerged leaves broader below the middle, without reticulation. 



7. P. heterophyllus. 

 Stipules adnate to the base of the linear-setaceous submerged leaves. 



8. P. diver si folius. 

 Species with submerged leaves only. 



Leaves with broad blades, lanceolate or oval, many-nerved. 



Leaves short-petioled or sessile, not amplexicaul. 9. P. lucens. 



Leaves more or less amplexicaul. 



Leaves elongate-lanceolate, semi-amplexicaul, cucullate; the straight apex of 



the embryo pointing to the base of the fruit. 10. P. praelongus. 



Leaves rounded-ovate to short-lanceolate, amplexicaul, not cucullate; the 

 curved embryo pointing inside the base of the fruit. 11. P. Richard sonii. 

 Leaves narrowly linear to capillary. 



Stipules free from the petioles and the leaf-blades. 

 Leaves 1.5-4 mm. wide. 



Species without glands at the base of the leaves; leaves with 3 principal 



nerves and several fine ones. 12. P. compressus. 



Species with glands at the base of the leaves. 



Glands large and translucent; nerves mostly 3; the curved end of the 



embryo pointing inside the base of the fruit. 13. P. obtusifolius. 



Glands small, dull; nerves of the leaves 5-7; the straight end of the 



embryo pointing to the base of the fruit. 14. P. Friesii. 



Leaves seldom more than 1.5 mm. wide, often less. 



Glands absent; nutlets keeled. 15. P. foliosus. 



Glands present; nutlets not keeled. 16. P. pusillus. 



Stipules adnate to the base of the leaves. 



Leaves 1.5 mm. wide or less, with entire margins. 



Stigma broad, sessile; nutlets indistinctly 1 -keeled or keel-less. 



Leaves filiform, less than 0.5 mm. wide; stipular sheaths 3-8 mm. long. 



17. P. filiformis. 

 Leaves about 1 mm. wide; stipular sheaths 1-2 cm. long. 



18. P. interior. 

 Stigma capitate, on an evident style; nutlets with 2 lateral but no median 



keel. 19. P. pectinatus. 



Leaves several-nerved, 3-6 mm. wide, finely serrulate seen under a lens. 



20. P. Robbinsii. 



