Robbins' pondweed, fern pondweed, potamot de Robbins

Rhizomes present. Cauline stems terete, without spots, to 100 cm; glands absent. Turions absent. Leaves submersed, conspicuously 2-ranked, sessile, stiffish; stipules persistent, conspicuous, adnate to base of blade about ± ¼ length of stipule, connate, greenish brown to white, ligulate, 0.5--2 cm, fibrous, shredding at tip, apex obtuse; blade dark green to reddish green, linear to lanceolate, not arcuate, 2--7(--12) cm ´ 3--4(--8) mm, base rounded, with basal lobes, not clasping, margins minutely spinulose to serrulate, not crispate, apex not hoodlike, acute, lacunae absent; veins 20--60, fine. Inflorescences often branched; peduncles not dimorphic, axillary, erect, cylindric, 3--5(--7) cm; spikes not dimorphic, moniliform (i.e., beaded), 7--20 mm. Fruits stipitate, brown, obliquely obovoid, turgid, abaxially and laterally keeled, 3--4(--5) ´ 2(--3.3) mm, lateral keels without points; beak erect, recurved at apex, 0.7--0.9 mm; sides without basal tubercles; embryo with less than 1 full spiral. 2n = 52.

Flowering late summer--early fall. Shallow to deep water of ponds, lakes, and slow-flowing rivers; 0--3000 m; Alta., B.C., Man., N.B., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask.; Ala., Alaska, Calif., Conn., Idaho, Ill., Ind., Maine, Mass., Mich., Minn., Mont., N.H., N.J., N.Y., Ohio, Oreg., Pa., R.I., Utah, Vt., Wash., Wis., Wyo.

Potamogeton robbinsii is our most easily recognized species when it is fertile. It is the only species with branched inflorescences. The species, however, occurs in fairly deep water, forming large colonies that essentially cover the substrate. Only rarely do the plants flower. It also is the only species with truly auriculate leaves, the blades forming small lobes projecting past the stem on each side of the stem. Leaf blades of other Potamogeton species may have slightly rounded bases, but no others have lobes that actually protrude past the stem.

The species has a fairly large disjunction; primarily known from the northern part of the flora, it also occurs in the Tensas River area, Baldwin County, Alabama. The Alabama population has been collected on at least two occasions over 40 years, once as recently as 1970.