456 NAIADACEJE. (PONDWEED FAMILY.) 
many-flowered, cylindrical. (P. Claytoni, Tuckerm. P. gramineus, 
Fries, L. ?) — Pools and shallow slow streams, common. — Floating 
leaves V or so in length, very variable. Nutlets roundish, flattened 
on the sides, obtuse and ridged on the back. 
6- hybridus, Michx. Floating leaves oval or lanceolate- 
oblong, 5- 7-nerved, on petioles rather shorter than the blade ; immers¬ 
ed leaves capillary (although flat under a lens) ; spike globular, Jew- 
flowered, on a short and somewhat club-shaped peduncle. (P. diver- 
sifblius, Barton. P. sethceum, Pursh .) — Shallow pools and streams, 
New England to Penn, and southward. — A delicate species, the 
thread-like branching stems 1° or so in length, and the floating leaves 
long, sometimes absent. Fruit nearly round, flattened on the 
sides, a little keeled and crested on the back. 
§ 2. Stipules free from the petiole or base of the leaf: leaves all im¬ 
mersed and similar, alternate, membranaceous, pellucid, nettcd-nerv- 
ed, lanceolate or broader: stems branching. 
7. P• 1 neons, L. Leaves oval-lanceolate, with a strong midrib 
and a small or tapering point, narrowed at the base into a more or less 
distinct petiole , the margin very minutely denticulate ; peduncles 
thickened upwards; spikes cylindrical, densely flowered; nutlets 
slightly keeled. — Ponds and deep streams. — A pretty large species, 
with the thin and shining leaves 3^ — 6* Jong by F or more broad, 
sometimes nearly sessile: stipules long and conspicuous. 
prael6ngus, Wulf. Leaves elongated-oblong, obtuse at 
both ends, partly clasping by the sessile base ; peduncles often much 
elongated; spike cylindrical, many-flowered; nutlets (when dry) 
strongly keeled on the back, short-pointed. —With No. 7 and 9, or 
in similar situations, and somewhat intermediate between the two. 
Leaves V or less wide, 2* — 7' long, often a little hooded at the blunt 
apex, the midrib not more conspicuous than two of the nerves. P 0 * 
duncle in deep water becoming 6' - 12' long. 
perfoliatus, L. Leaves clasping by a heart-shaped bast, 
ovate or ovate-lanceolate, sometimes round-ovate, obtuse ; spikes rather 
few-flowered ; nutlets rounded on the back (when fresh), very short- 
pointed. Ponds and rivers, common. — Leaves shining, about V 
long, flat ) or, in the longer and ovate-lanceolate American forms in¬ 
clined to be acute and more or less wavy or crisped. — J°. crxspum 
probably has not been found in this country. 
§ 3. Stipules free from the petiole : leaves all immersed and similar, al¬ 
ternate, grassy-linear or thread-like, sessile : stems much branching- 
10. P. couapressus, L. ex Fries. Stem very flat, nearly the 
breadth of the narrowly linear abruptly pointed leaves ; spikes cylindri¬ 
cal, 10 — 15 -flowered ; nutlets rounded-obovate, lenticular, keeled, tip* 
ped with a short terminal point. (P. zostersefblius, Schum .) — P° nds ’ 
