486 NAIADACE^E. (PONDWEED FAMILY.) 



about equalling the rather dilated 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 ('- l' long, ^"-|" wide) ; stipules early lacerate; 

 submersed flowers 'usually solitary on very short erect peduncles ; shell-shaped fruit with 

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

 coiled 1 3 turns. Rivers, and even far up small streams, in company with the 

 No. 3, or rarely with the following, Maine (J. Blake) to Lake Superior and 

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



6. P. h^bridus, Michx. Floating leaves varying from oval to lance- 

 oblong ; (the largest 10" long and 6" wide), often acute, longer than t\\e 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 1^ 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, 11- 

 1 7-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-petiokd, 17-23- 

 nerved; submersed leaves very 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, Chamisso?) 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. Chamisso's name of this and No. 3 merely provisional. 



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

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

 or ovate-oblong, 25 -37 -nerved; all alternate; upper submersed ones (3-5) usually 

 lanceolate, acute at the base and very long-acuminate, 10-15-nerved, 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 spatulate- 

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

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



