Aquatic and Wetland Vascular Plants of the Northern Great Plains
55. Potamogetonaceae, the Pondweed Family
1. Potamogeton L. -- Pondweed18. Potamogeton vaginatus Turcz. -- Sheathed pondweed
Stems terete, mostly 1-2 mm thick, freely branched above, to 1.5 m long. Leaves all submersed, filiform to narrowly linear, 2-8(30) cm long, 0.5-2 mm wide, 1(3)-nerved, the tip acute to obtuse or sometimes retuse; stipules adnate to the base of the leaf blade for 1-5 cm and sheathing the stem, the sheaths along the main stem inflated 2-5X the thickness of the stem. Spikes elongate, 3-6 cm long, with 5-12 evenly spaced floral whorls; peduncles slender and lax, to 10 cm long, often much surpassed by the upper leaves. Fruits dark green, obliquely obovoid, ca. 3 mm long, rounded on the back; stigma sessile, forming a low beak. Jul--Aug. Deep water of cold, clear lakes; occasional in the Turtle Mts., ND, otherwise collections from Day Co., SD and Crook Co., WY; (Newf. to AK, s to NY, WI, MN, SD, WY and OR; also Eurasia and Africa).
Deep water forms of P. pectinatus, especially those from wave swept
areas, are often confused with P. vaginatus. In flowering or fruiting
condition the two species are easily discerned by the number of floral whorls
per spike and the shape of the fruit (beaked vs. nearly beakless). Unfortunately,
deep water forms of P. pectinatus often do not flower and they tend
to have stout stems sheathed by prominent, somewhat inflated stipules similar
to P. vaginatus. The latter tends to have longer, more filiform leaves
(much surpassing the spikes) than deep water forms of P. pectinatus,
and the stipules are more prominently inflated.
Previous Section -- Potamogeton strictifolius Benn.
Return to Family -- Potamogetonaceae - The Pondweed Family
Next Section -- Potamogeton zosteriformis Fern. -- Flatstem pondweed
