4. Potamogeton bicupulatus Fernald, Memoirs of the american academy of arts and science. 17: 112. 1932.
Potamogeton diversifolius Rafinesque var. trichophyllus Morong
Rhizomes present. Cauline stems compressed, without spots, 10--25 cm; glands absent. Turions absent. Leaves both submersed and floating or floating absent, ± spirally arranged. Submersed leaves sessile, lax; stipules persistent to deliquescent, inconspicuous, convolute, adnate to blade for less than ½ stipule length, light green, ligulate, 0.2--1.2 cm, not fibrous, not shredding at tip, apex obtuse; blade light green to rarely brown, linear-setaceous, not arcuate, 1.5--11 cm ´ 0.1--0.4(--0.6) mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, tapering, lacunae absent; veins 1. Floating leaves petiolate; petioles continuous in color to apex, 5--35 mm; blade adaxially light green, lanceolate-elliptic to broadly elliptic, 0.6--2.3(--2.8) cm ´ 1--11 mm, base tapering or rounded, apex acute to long tapering; veins 3--7. Inflorescences unbranched; peduncles dimorphic, submersed axillary, somewhat recurved, clavate, 1--10 mm, emersed axillary or terminal, erect to slightly recurved, slightly clavate, 3.5--22 mm; spikes dimorphic, submersed , globular to ellipsoid, 1.5--7 mm, emersed ellipsoid to cylindric, 3--14 mm. Fruits sessile, greenish brown, somewhat orbicular, compressed, abaxially keeled, laterally keeled, 1.1--2.1 ´ 1.1--2 mm, lateral keel without points; beak absent; sides without basal tubercles; embryo with more than 1 full spiral.
Flowering early summer--fall. Acidic waters of ponds, lakes, and streams; 0--300 m; Ont.; Conn., Del., Ind., Maine, Mass., Mich., N.H., N.Y., Pa., R.I., Tenn., Vt., Va., Wis.
Potamogeton bicupulatus is an uncommon species of the acid lakes and streams of northeastern United States and southern Canada. It is the final third species we have with dimorphic inflorescences and embryos with more than one full spiral. It can be separated from the other two, Potamogeton spirillus and P. diversifolius, because it has very narrow submersed leaves without lacunae and fruits with lateral keels without sharp points.