NAIADACEAE. 47 



wide, acute, obtuse or cuspidate at the apex, mostly 5 -nerved, rarely 7-nerved, 

 2-glandular at the base, the glands small; stipules white, hyaline, finely nerved, 

 obtuse or acute, 1-2 cm. long; peduncles often thicker than the stem and some- 

 times thickening upward; spikes, when developed, interrupted; fruit quite similar 

 to that of P. pusillus, but with a recurved style, usually with a shallow pit on the 

 sides, and with the apex of the embryo pointing toward the basal end. In still 

 water, N. B. to N. Y., west to Br. Col. Also in Europe. Propagating buds occa- 

 sional. July-Aug. 



24. Potamogeton rutilus Wolfg. Slender Pondweed. (I. F. f. 165.) 

 Stems very slender, 0.2-0.6 m. long, compressed, simple or nearly so. Leaves 

 2-3.5 cm - l° n g> °-$- Y mm - wide, acute or acuminate, strict, nearly erect, 3-5- 

 nerved, revolute, often 2-glandular at base and bright green, the nerves prominent 

 beneath ; stipules acute, 1-2 cm. long, often longer than the internodes and hiding 

 the bases of the leaves above, persistent, becoming white and fibrous with age; 

 peduncles 1-3.5 cm. long; spikes 6-10 mm. long, usually dense, but sometimes 

 interrupted; fruit obliquely obovoid, about 2 mm. long and I mm. thick, obscurely 

 keeled or the back showing only 2 small grooves ; apex of the drupelet tapering 

 into a short facial nearly straight recurved style ; embryo circle not complete, the 

 apex pointing a little inside of the base. Anticosti and James Bay to Mich, and 

 Minn. Also in Europe. 



25. Potamogeton Vaseyi Robbins. Vasey's Pondweed. (I. F. f. 166.) 

 Stems filiform, widely branching below, and with many short lateral branches 

 above, 0.3-0.5 long, the emersed fertile forms in shallow water, and the more com- 

 mon sterile submerged forms in water from 2-3 m. in depth. Floating leaves on 

 the fertile stems only, coriaceous, in 1-4 opposite pairs, oval, oblong or obovate, 

 8-10 mm. long, 4-6 mm. wide, with 5-9 nerves deeply impressed beneath, tapering 

 at the base into petioles 6-8 mm. long; submerged leaves capillary, 2-4 cm. long; 

 stipules white, delicate, many-nerved, acute or obtuse, 4-6 mm. long; peduncles 

 6-12 mm. long, thickening in fruit ; spikes 4-6 mm. long, 2-6 fruited ; fruit 

 roundish-obovoid, about 2 mm. long and nearly as thick, 3 -keeled, the middle keel 

 rounded, tipped with a straight or recurved style. E. Mass. to S. Ont. and O. 

 The plant is furnished with propagative buds. July-Aug. 



26. Potamogeton lateralis Morong. Opposite-leaved Pondweed. (I. F. 

 f. 167.) Stems filiform, much branched. Floating leaves on sterile shoots only, 

 coriaceous, elliptic, obtuse, 8- 10 mm. long, 2-4 mm. wide, 5-7 -nerved, the nerves 

 deeply impressed beneath, usually in 1-3 opposite pairs which stand at right 

 angles to the stem on petioles 6-20 mm. long; submerged leaves linear, acute, 2-8 

 cm. long, 0.5-1 mm. wide, 1-3 -nerved, 2-glandular at the base, but the glands 

 small and often obsolete ; stipules small, hyaline, many-nerved, obtuse, deciduous; 

 peduncles and floating leaves lateral, widely spreading at maturity, sometimes re- 

 curved, spikes capitate or often interrupted, 3-4-flowered; fruit obliquely obovoid, 

 about 2 mm. long, lenticular, the back much curved and 2 -grooved, the face arched 

 and surmounted by the nearly sessile stigma; curve of the embryo oval, its apex 

 nearly touching its base. In lakes and slow streams, E. Mass. to Mich. Prolifer- 

 ous shoots at the summit of the stem and on the upper branches appear late in the 

 season, as the plants are beginning to decay. July-Aug. 



27. Potamogeton pusillus L. Small Pondweed. (I. F. f. 168.) Stems 

 filiform, branching, o. 1-0.6 m. long. Leaves all submerged, linear, obtuse and 

 mucronate or acute at the apex, 2-glandular at the base, 2.8 cm. long, about I mm. 

 wide, 1-3 -nerved, the lateral nerves often obscure; stipules short, hyaline, obtuse; 

 peduncles usually 6-18 mm., or rarely 7.5 cm. long; spikes 3- 10- flowered; fruit 

 obliquely ellipsoid, about 2 mm. long and 1 mm. thick, curved and 2-grooved on 

 the back or sometimes with 3 distinct keels, the face slightly arched, beaked by a 

 straight or recurved style; apex of the embryo slightly incurved and pointing ob- 

 liquely downward. Propagative buds occur in greater or less abundance. In 

 ponds and slow streams, N. B. to Br. Col., south to Va., Tex., and Cal. Also in 

 Europe. July-Aug. 



Potamogeton pusfllus Panormitknus (Biv.) Morong. Uppermost leaves subcoriace- 

 ous, spatulate, opposite, divaricate, 3-5-nerved, 8-10 mm. long, tapering into a broad 

 petiole as long as the blade. Ottawa, Ont. Also in Europe, 



