Aquatic and Wetland Vascular Plants of the Northern Great Plains

55. Potamogetonaceae, the Pondweed Family

1. Potamogeton L. -- Pondweed

15. Potamogeton pusillus L. -- Small pondweed, baby pondweed

Stems terete to subterete, 0.1-0.7 mm thick, simple to freely branching, 2-15 dm long. Leaves all submersed, linear, 0.9-6.5 cm long, 0.2-2.5 mm wide, 1- to 3(5)-nerved, acute to obtuse or apiculate at the tip, tapered to the sessile base; stipules free, brownish-green, 3-9 mm long, delicate and nonfibrous, soon decomposing; glands usually present at the base of the stipules. Winter buds sometimes produced, lateral or terminal, 0.9-3.2 cm long; inner leaves rolled into an indurate fusiform structure; outer leaves 1-3 per side, acute to obtuse, without corrugations at the base. Spikes short-cylindric to cylindric, 1.5-10 mm long; floral whorls 1-3(4), 1.2-4.7 mm apart; peduncles filiform to cylindric, 0.5-6 cm long. Fruits green to brown, obliquely obovoid, 1.5-2.2 mm long, rounded on the back, often concave on the sides. Jun--Aug. Shallow to deep water of lakes, ponds, marshes, ditches and streams, where water is fresh to brackish; common and often abundant; (Newf. to N.W.Terr., s throughout most of the U.S. and into Mex.; also Eurasia).

Two varieties, var. tenuissimus Mert. & Koch (P. berchtoldii Fieb.) and var. pusillus, occur in the region. By far the prevalent form is var. pusillus, characterized as follows: Leaves with up to 2 rows of lacunae along the midrib, apex acute, rarely apiculate; stipules mostly connate. Spikes usually of 2-4 verticels; peduncles filiform to cylindric, usually 1-3 per plant. Mature fruit widest above the middle, the sides concave, the beak positioned forward.

The var. tenuissimus is of limited occurrence in this region, occurring infrequently in the e part. It differs from var. pusillus as follows: Leaves with 1-5 rows of lacunae along the midrib, apex acute to obtuse; stipules mostly convolute. Spikes mostly of 1-2 adjacent verticels; peduncles cylindric, usually more than 3 per plant. Mature fruit mostly widest at or below the middle, the sides rounded, the beak positioned centrally.

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