Aquatic and Wetland Vascular Plants of the Northern Great Plains
55. Potamogetonaceae, the Pondweed Family
1. Potamogeton L. -- Pondweed4. Potamogeton diversifolius Raf. -- Waterthread pondweed
Stems slender, terete, 0.5-1 mm thick, to 8 dm long, usually with short lateral branches. Submersed leaves linear, flat, 1-8 cm long, 0.3-1.5 mm wide, 1(3)-nerved, obtuse to long-acuminate; stipules of submersed leaves 2-18 mm long, membranous, adnate to the leaf blade for mostly 1-4 mm, usually adnate for less than 1/2 the total length. Floating leaves sometimes lacking, the blades elliptic to elliptic-lanceolate or elliptic-oblanceolate, 5-40 mm long, 5-20 mm wide, 3- to 17-nerved, acute to rounded at the tip, cuneate to rounded at the base; petioles mostly 5-40 mm long; stipules of floating leaves free of the leaf bases, 2-25 mm long, membranous to weakly fibrous. Spikes dimorphic, the lower submersed spikes capitate to ellipsoid, 1.5-6 mm long, few- to several-fruited, the upper spikes ellipsoid to cylindric, 5-30 mm long, usually many-fruited; peduncles slightly clavate, 3-32 mm long. Fruits olive to yellowish, round and flattened, with a prominent winged dorsal keel and slightly ridged to winged lateral keels, the keels entire to toothed, the embryo coil plainly visible through the papery thin walls of the fruit, the beak minute. Jun--Sep. Shallow water of ponds and marshes; uncommon and scattered; (CT to MT and OR, s to FL, TX, AZ, CA and into Mex.).
Earlier reports of P. spirillus Tuckerm. for ND were based on misidentified specimens of P. diversifolius.
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