NAIADACEJE. (PONDWEED FAMILY.) 563 



the face arching and terminated by the short style ; summit of the large embryo 

 lying transverse to the fruit. (P. compressus, Fries ; not L. ?) Still and slow- 

 flowing waters, N. Eng. to N. J., Iowa, and Minn. Aug., Sept. (Eu.) 



20. P. Hillii, Morong. Stem slender, widely branching , Jlattish ; leaves 

 linear, acute (1 -2|' long by |- 1" wide), 3-nerved, the lateral nerves delicate 

 and near the margin ; stipules whitish, striate, obtuse (3 - 5" long) ; spikes 

 capitate (3-6-fruited), on short spreading or recurved peduncles; fruit as in 

 the last. Mich, and western N. Y. 



21. P. Obtusifdlius, Mertens & Koch. Stem flattened, very branching ; 

 leaves linear, tapering toward the base, obtuse and mucronate or very acute, 

 3- (rarely 5-) nerved ; stipules elongated, very obtuse; spike ovate, continuous, 

 5 - 8-flowered, about the length of the peduncle ; fruit oval, apiculate with the 

 style, not keeled when fresh, upper portion of embryo coiled inward and lying 

 transverse to the fruit. Slow streams and ponds, Canada and N. Eng. to west- 

 ern N. Y. and Mich. Sept., Oct. (Eu.) 



22. P. paucifldrus, Pursh. Stem filiform, Jlattish and very branching ; 

 leaves narrowly linear (1-2' long and seldom |" wide), acute, obscurely 3- 

 nerved; stipules obtuse; spikes capitate, 1-4- (usually 2-) flowered, on short 

 club-shaped peduncles ; fruit roundish -lenticular ; the back more or less crested ; 

 upper portion of the embryo incurved in a circle. Still or stagnant waters, 

 N. Brunswick to Ga., Iowa, Minn., and westward. 



Var. Niagar&nsis, Gray. Stem often longer (1 - 3) ; leaves larger ( 1| - 

 3^' long by 1" wide or less), 3-5-nerved at base, very acute and mucronate, 

 narrowed to the subpetiolate base. (P. Niagarensis, Tuckerm.) Running 

 water, Great Lakes to S. C. ; also in Cal. 



23. P. pusillus, L. Stem slender, flattish or nearly cylindrical, often very 

 branching ; leaves narrow- or setaceous-linear, acuminate, acute or subacute, 1 - 

 3-nerved, furnished with translucent glands on each side at the base ; stipules at 

 first obtuse ; spikes interrupted or capitate, 2 - 8-flowered, on rather long pe- 

 duncles ; fruit obliquely elliptical, scarcely keeled ; apex of embryo incurved and 

 directed obliquely downward. Pools and ditches, N. Scotia to N. J., west to 

 Minn, and Mo., and westward. Leaves sometimes almost setaceous (var. 

 tenuissimus, Koch). 



Var. polyph^llus, Morong. Dwarf form (3 - 5' high), divaricately branch- 

 ing from the base, very leafy throughout ; leaves very obtuse, not cuspidate, 

 3-nerved ; non-flowering but abundantly provided with propagating buds which 

 are formed on the thickened and hardened ends of the branches and closely 

 invested by imbricated leaves. In a shallow pool, S. Natick, Mass. 



24. P. mucronatus, Schrad. Resembling P. pusillus, but stem less 

 branching; leaves broader (almost 1" wide), often 5-nerved ; spikes interrupted. 

 (P. pusillus, var. major, Fries.) N. Brunswick to western N. Y., Mich., and 

 Minn. July. (Eu.) 



25. P. gemmiparus, Robbins. Stem filiform, branching, terete, varying 

 greatly in height ; leaves hair-like, sometimes not as broad as the stem, often 

 with no apparent midrib, tapering to the finest point (1 - 3' long), bi-glandular at 

 base; stipules -J- 1' long; spikes few (3-6-flowered), interrupted, on long fili- 

 form peduncles ; propagating buds very numerous ; fruit like that of P. pusillus, 

 very rare. (P. pusillus, var.? gemmiparus, Robbins.) Slow-moving streams 

 and still water, Mass. Aug., Sept, 



