NAIADACEJE. (PONDWEED FAMILY.) 487 



style, distinctly 3-keeled when fresh, sharply so when dry ; nutlet marked on 

 the back by two deep furrows and in front by a sinus below the angle ; sides 

 flat ; upper portion of the embryo circularly much incurved. — Ponds ; found as 

 yet only in Eastern Massachusetts and " in ponds on hills north of St. Louis," 

 Missouri, Engelmann : also Georgia, Le Conte. — July, Aug. 



10. P. amplif61ius, Tuckerman. Stems simple, of very variable length; 

 floating leaves (sometimes wanting) large, oblong or lance-ovate, sometimes slightly 

 cordate at base, abruptly acutish, 30 - 50-nerved, on rather long petioles ; sub- 

 mersed leaves often very large (reaching 7' by 2'), lanceolate or oval, acute at 

 each end, usually much recurved, undulate, mostly on short petioles ; stipules 

 very long and tapering to a point, soon becoming loose ; peduncles thickened up- 

 ward, in deep water much elongated ; fruit rather obliquely obovate, the back 

 rounded and bluntly keeled ; nutlet slightly impressed on the sides ; upper 

 portion of the embryo incurved into a ring. — Ponds and large rivers : not rare. 



— Aug., Sept. — In very shallow water sometimes without membranaceous 

 leaves, and in deep water it may have those only. — (Leaves on a radical 

 shoot in one specimen with adnate stipules !) 



11. P. gramineilS, L. Stem slender, very branching below ; floating leaves 

 mostly thin, variable, but with a short blunt point, 9-15-nerved; submersed 

 ones usually lanceolate or linear-lanceolate, acuminate or cuspidate, narrowed 

 towards the base, about 7-nerved on the stem and 3-nerved on the branches ; 

 upper ones petioled, lower sessile; stipules obtuse, loose; peduncles somewhat 

 thickened upwards; fruit small, roundish, compressed, scarcely keeled ; upper 

 portion of the embryo annular. (P. heterophyllus, Schreber.) — Still or flowing 

 water : common. — Varies exceedingly in its submersed leaves, peduncles, and 

 otherwise. The ordinary forms are: — Var. graminif6lius, Fries. Sub- 

 mersed leaves lance-linear, attenuate at each end, flaccid, sometimes more 

 than 4' long by 3" wide ; stem elongated. — Var. heterophyllus, Pries. 

 Lower leaves shorter, lanceolate, more rigid : the commonest form. The follow- 

 ing are doubtfully referred to this species. (Eu.) 



Var.'? spathulsefdrmis. (P. spathseformis, Tuckerman, in herb.) Branches 

 scattered ; floating leaves obovate or oblong, with a larger point ; submersed 

 ones spatulate-oblong, obtuse, mucronate, sometimes recurved ; spikes large and 

 densely flowered. — Mystic Pond, near Boston, Tuckerman. The fruit is lacking 

 to prove its rank. 



Var. ? myrioph^llus. Sending up from running rootstocks many short 

 repeatedly dichotomous and densely leafy stems ; fertile stems very slender ; 

 floating leaves small, delicate, lance-oblong, on long filiform petioles ; sub- 

 mersed stem-leaves larger, early perishing ; those of the branches (deep green) 

 linear-oblanceol ate, very small (|'-l'long), acute, sometimes minutely serru- 

 late ; spike slender, loosely-flowered, much shorter than the thickened peduncle. 



— Apponaug Pond, Rhode Island, without fruit. 



§ 2. Conformifolii. Leaves all submersed and similar, mostly sessile, membrana- 

 ceous and dilated, lanceolate, oblong, or oval. (Stipides obtuse, becoming loose.) 



12. P. lticens, L. Stem thick, branching, sometimes very large; leaves 

 more or less petioled, oval or lanceolate, mucronate, often rough-serrulate, frequmtlg 



