455 
NAIADACEAL (PONDWEED FAMILY.) 
* Stem rather large, usually simple. 
1. P. natans, L. Leaves all long-pelioled ; the floating ones 
coriaceous, oval, elliptical, or ovate, chiefly rounded or a little heart- 
shaped at the base, many-nerved ; the immersed ones linear or lanceo¬ 
late, or thread-form (the blade often evanescent by decay) : nutlets 
short-pointed, rounded on the back when fresh , more or less keeled when 
dry (2" long or more).-\ ar. 1. fluitans (P. fluitans, Roth.), has 
narrower oblong-lanceolate or oval leaves, often tapering at the base 
the lower more pellucid; fresh nutlets acutish on the back._Ponds 
and slow streams. July - Sept. — Floating leaves 2'- 3' long. 
2. P, oblungus, Viv., Fries. Leaves, &c., nearly os°in No 1 
but mostly narrower; nutlets small (V long), obtuse and pointless, 
always rounded on the back. — Pools and ditches, apparently common 
as m N. Europe. — Floating leaves usually oblong-elliptical or oblong- 
lanceolate : the rounded fruits not half as large as in the true P. na¬ 
tans. 
ptilclier, Tuckerm. Floating leaves ovate or broadly 
oval, rounded or heart-shaped, or the lower tapering at the base, on a 
petiole about the length of the blade ; immersed leaves large, short-peti- 
oled, broadly lanceolate or oblong, tapering to both ends, wavy ; “ nut¬ 
lets turgid, lunate, acutely keeled on the back.”—Ponds, common 
through the Northern States. — Whether I am describing the same 
plant as Mr. Tuckerman is uncertain, as I have not the ripe fruit: 
but this apparently well-marked species deserves the name. It is 
quite as large as No. 1; the floating leaves (some of them membra¬ 
nous) often longer than their petioles, 3'-4' long by 1£' wide, but much 
exceeded in size by the principal immersed ones, which are 5'-8 ; 
long by 2' or more in width, very pellucid, somewhat taper-pointed, 
all but the uppermost gradually narrowed at the base into a margined 
petiole 1' or less in length. It seems to be most allied to P. plantagi- 
neus and P. spathulatus. 5 
4* P» rufesccns, Schrader. Floating leaves oblong or obo- 
vate, narrowed into a petiole shorter than the blade, only the upper a 
little coriaceous; immersed leaves all sessile or nearly so, lanceolate, 
flat, mostly obtuse, tapering to the base; nutlets acutely keeled, tip¬ 
ped with a small pointed beak. (P. fluitans, Smith , &c.) —Slow 
streams, &c., northward. — More slender than any of the preceding. 
Leaves often purplish ; the immersed ones 2' -4' long, about A' wide 
pellucid. ’ 
* * Stems slender or thread-like, usually branched below. 
^HuSj Schreber. Floating leaves elliptical 
or oblong, or the lowest lanceolate-spatulate, long-petioled; immersed 
oiies lanceolate or linear, sometimes elongated and grass-like, flaccid, 
o scurely denticulate or roughish on the margins, the lower sessile; 
pe unc es much thicker than the stem, elongated (l f -2' long); spike 
