674 NAIADACE^E POTAMOGETON 



7 POTAMOGETON L. Sp. 126. 



Submersed aquatic herbs with slender jointed branching stems, 

 mostly alternate leaves with scarious stipules and perfect flowers 

 in peduncled axillary spikes. Perianth herbaceous, of 4 narrow 

 valvate segments. Stamens 4, opposite to the segments, with 

 nearly sessile 2-celled anthers. Ovaries usually 4 and sessile, 

 with oblique depressed nearly sessile stigmas and solitary ascend- 

 ing campylotropous ovules. Fruit somewhat compressed, ovate, 

 drupe-like, with a crustaceous nutlet within. Seed with mem- 

 branous testa and strongly curved or spiral embryo. 



* Floating leaves more or less coriaceous, with a dilated petioled 

 blade, different in form from the thinner submerged ones : stipules 

 free : spikes cylindrical, mostly dense, not interrupted. 



P. natans L. Sp. 126. Stems 1-4 feet long, simple or sparingly bran- 

 ched: floating leaves thick, ovate-elliptic to lanceolate, acutish, slightly 

 cordate at base, usually 2-3 inches long, mostly shorter than the petiole, 

 2 1-29- nerved: stipules long and conspicuous, acute or acuminate ; upper 

 submersed leaves often with a small lanceolate blade, the lower reduced to 

 petioles : peducles stout, bearing an emersed spike 1-2 inches long : fruit 

 turgid, obliquely obovate, acute, 2 lines long: nutlets with a small deep 

 pit on each side : embryo nearly circular. In ponds and ditches, Alaska to 

 California and across the Continent: olso in Europe and Asia. 



P. amplifolius Tuckerm. Am. Journ. Sci. (II) vi, 225. Stems often 

 stout, simple: floating leaves elliptic to oblong-lanceolate, acute, mostly 

 rounded or slightly cordate at base, 2-4 inches long, 30-50-nerved, on peti- 

 oles about as long as the blade ; stipules large and conspicuous : submerged 

 leaves often very large, mostly falcate and somewhat undulate, acute, at- 

 tenuate to a usually short petiole : spikes thick and often dense, 1-3 inches 

 long, on very short peduncles: fruit over 2 lines long, 3-keeled, with a 

 broad stout beak : sides of the nutlet not pitted : embryo slender, the coty- 

 ledon incurved. In ponds and streams, Brit. Columbia to California and 

 the Eastern States. 



P. pnlcher Tuckerm. Am Journ. Sci. xlv, 38. Stems simple, terete, 

 black-spotted, 1-2 feet long : floating leaves usually massed at the top on 

 short lateral branches, ovate or round-ovate, subcordate, 2-5 inches long, 

 many-nerved : petioles about as thick as the stem, 2-4 inches long, spotted: 

 submerged leaves of two kinds, the uppermost lanceolate, long-acuminate, 

 undulate, 3-8 inches long, 6-8 lines wide, tapering into a short petiole, the 

 lowest much thicker, spatulate, oblong or ovate, on petioles %-4 inches 

 long: stipules 2-carinate: spikes dense, long-peduncled : fruit adout 2 lines 

 long, turgid, tapering into & stout apical style, the back sharply 3-keeled : 

 embryo coiled. In ponds, Idaho to Main and Georgia. 



P. Xuttallii Cham. & Sch. Linn, ii, 226. P. Claytonii Tuckerm. 

 Stems compressed, mostly simple, 2-6 feet long: floating leaves narrowly 

 oblong to elliptic, 1-3 inches long, 11-17-nerved, obtuse or acutish, attenu- 

 ate below into a flattened petiole usually shorter than the blade : stipules 

 sheathing, soon deciduous, an inch or less long : submerged leaves very 

 thin .linear, 2-5 inches long, 5 nerved, with a close cellular reticulation 

 between the middle nerves ? ppikes 6-12 lines long, on short stout peduncles: 

 fruit obovate, 3-keeled, slightly apiculate, 1-1 ^ lines long: nutlets slightly 

 depressed on the sides : embryo coiled nearly \% times. In ponds and 

 streams, California to Alaska and the Eastern States. 



P. alpinns Balbis Miso. Bot. 13. (1804). P. rufe&cen* Schrad. (1816.) 



