486 NAIADACE.E. (PONDWEED FAMILY.) 



about equalling the rather diluted petioles, with 5 - many nerves beneath deeply 

 impressed ; upper submersed leaves either with or without a lance-oblong or 

 broad-linear proper blade; the numerous lower ones narrow-linear, tapering 

 towards the obtuse apex {%' - 1^' long, J"-|" wide) ; stipules early lacerate; 

 submersed /lowers usually solitary on very short erect peduncles ; shell-shaped fruit with 

 the buck either winged and with 4-5 distinct teeth or wingless and entire; embryo 

 coiled 1% turns. — Rivers, and even far up small streams, in company with the 

 preceding, rarely with the following, Maine (J. Blake) to Lake Superior and 

 Virginia. June - Aug. — Stem less slender than the next. 



6. P. hybridus, Michx. Floating leaves varying from oval to lance- 

 oblong ; (the largest 10" long and 6" wide), often acute, longer than the, filiform 

 petioles, with about 5-7 nerves beneath deeply impressed ; submersed leaves very 

 numerous, almost setaceous, (l'-3' long, very rarely |" wide) ; stipules obtuse; 

 submersed spikes 1 -4-flowered, their peduncles (of their own length) frequently 

 recurved ; fruit minute, about 8-toothed on the margin; embryo coiled lj turns. (P. 

 diversifolius, Barton. P. setaceus, Pursh.) — Shallow stagnant waters. June- 

 Aug. — Emersed spikes 4" to (in var. spicatus, Engelm.) 7" long. 



* * Submersed leaves lanceolate, rarely oval or linear, membranaceous. 



7. P. rufescens, Schrader. Stem simple, floating leaves (often wanting, 

 P. obrutus, Wood) rather thin, wedge-oblanceolate, narrowed into a short petiole, 1 1 - 

 17-nerved; submersed leaves almost sessile, lanceolate and lance-oblong, smooth 

 on the margin, fewer-nerved ; stipules broad, hyaline, obtuse, upper ones 

 acuminate ; fruit obovate, lenticular, pitted when immature, with an acute 

 margin and pointed with the rather long style ; embryo incompletely annular. 

 — Rivers and streams, New England to Pennsylvania, Illinois, and especially 

 northward. — Aug. - Sept. — Spike often somewhat compound ! (Eu.) 



8. P. lonchites, Tuckerman. Stem often branching below ; floating 

 leaves thinnish, lance-oblong or long-elliptical, often acute, long-petioled, 17-23- 

 nerved; submersed leaves vei-y long (3' -12', by 2" -12" wide), lanceolate and 

 lance-linear, 7-15-nerved, coarsely reticulated ; peduncles somewhat thickened 

 upwards ; fruit obliquely obovate, obscurely 3-keeled when fresh and distinctly 

 so when dry, the middle one winged above and sometimes with 3-5 shallow 

 indentations ; the rounded slightly curved face surmounted by the short style; 

 nutlet with the sides scarcely impressed ; upper part of the embryo circularly 

 incurved. (P. Americanus, Chumisso?) — Rivers and ponds, New England to 

 Illinois. Aug., Sept. — More slender than the very similar P. fluitans of 

 Europe, its leaves longer petioled, the floating ones more abrupt at the base; 

 stipules not bicarinate. 



9. P. pillcher, Tuckerman. Stem simple, black-spotted ; leaves of three 

 kinds ; floating ones becoming very large (4 17 by 3|') roundish-ovate and cordate 

 or ovate-oblong, 25 - 3 7 -nerved ; all alternate; upper submersed ones (3-5) usually 

 lanceolate, acute at the base and \cry long-acuminate, 10-15-ncrved, very thin, 

 cellular each .side of the midrib, undulate, short-petioled ; lowest (2-4 near the 

 base of the stem) thicker, plane, oval or oblong with a rounded base, or spatulatc- 

 efelong, on longer petioles; stipules rather short and obtuse; peduncles thicker 

 than the stem ; fruit with a rounded back and angular face, pointed with the 



