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fern pondweed, potamot de Robbins, Robbin's pondweed

diverse-leaf pondweed, Rafinesque's pondweed, water-thread pondweed

Rhizomes

present.

present.

Cauline stems

terete, without spots, to 100 cm;

glands absent.

compressed, without spots, 10–35 cm;

glands absent.

Turions

absent.

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.

both submersed and floating or floating absent, ± spirally arranged.

Floating leaves

petiolate;

petioles continuous in color to apex, 0.7–0.8 cm;

blade adaxially light green, obovate to elliptic, 0.8–1.6 cm × 3–8.5 mm, base acute, apex round to acute;

veins 3–7.

Submersed leaves

sessile, lax;

stipules persistent to deliquescent, inconspicuous, convolute, adnate to blade ½ stipule length, light brown to red-brown, ligulate, 1.5–2.3 cm, not fibrous, not shredding at tip, apex obtuse;

blade red-brown to light green, linear, often arcuate, 1–1.3 cm × 0.1 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, acute, lacunae present, 1–2 rows each side midrib;

veins 1.

Inflorescences

often branched;

peduncles not dimorphic, axillary, erect, cylindric, 3–5(–7) cm;

spikes not dimorphic, moniliform (i.e., beaded), 7–20 mm.

unbranched;

peduncles dimorphic, submersed axillary, recurved, clavate, 3–5 mm, emersed axillary or terminal, erect to slightly recurved, clavate, 6–15 mm;

spikes dimorphic, submersed capitate, 2–3 mm, emersed cylindric, 5–9.7 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.

sessile, greenish brown, orbicular, compressed, abaxially winged, laterally winged, 1–1.5 × 0.9–2 mm, lateral wings with sharp points;

beak present, erect, 0.1 mm;

sides without basal tubercles;

embryo with more than 1 full spiral.

2n

= 52.

Potamogeton robbinsii

Potamogeton diversifolius

Phenology Flowering late summer–early fall. Flowering and fruiting spring–fall.
Habitat Shallow to deep water of ponds, lakes, and slow-flowing rivers Ponds, lakes, streams, and rivers
Elevation 0–3000 m (0–9800 ft) 5–2500 m (0–8200 ft)
Distribution
from FNA
AK; AL; CA; CT; ID; IL; IN; MA; ME; MI; MN; MT; NH; NJ; NY; OH; OR; PA; RI; UT; VT; WA; WI; WY; AB; BC; MB; NB; NS; NT; NU; ON; PE; QC; SK
[WildflowerSearch map]
[BONAP county map]
from FNA
AL; AR; AZ; CA; CO; CT; DE; FL; GA; IA; ID; IL; IN; KS; KY; LA; MA; MD; MN; MO; MS; MT; NC; ND; NE; NJ; NV; NY; OH; OK; OR; PA; SC; SD; TN; TX; VA; VT; WI; WV; Mexico; Wyo
[WildflowerSearch map]
[BONAP county map]
Discussion

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.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Potamogeton diversfolius is very likely the most common species of Potamogeton the genus in the southeastern United States. It has been separated into two species, P. diversifolius in the strict sense and P. capillaceus (M. L. Fernald 1932). The species has also been divided into two varieties, var. diversifolius and var. trichophyllous Morong (D. O S. Correll and M. C. Johnston 1970) . Potamogeton diversifolius var. trichophyllous actually is misapplied, as the name really refers to the more northern P. bicupulatus Fernald. We are following E. J. Klekowski Jr. and E. O. Beal (1965) in accepting only one taxon, as we have studied the species over much of its range and reached similar conclusions.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Source FNA vol. 22. FNA vol. 22.
Parent taxa Potamogetonaceae > Potamogeton Potamogetonaceae > Potamogeton
Sibling taxa
P. alpinus, P. amplifolius, P. bicupulatus, P. clystocarpus, P. confervoides, P. crispus, P. diversifolius, P. epihydrus, P. floridanus, P. foliosus, P. friesii, P. gramineus, P. groenlandicus, P. hillii, P. illinoensis, P. natans, P. nodosus, P. oakesianus, P. oblongus, P. obtusifolius, P. ogdenii, P. perfoliatus, P. praelongus, P. pulcher, P. pusillus, P. richardsonii, P. spirillus, P. strictifolius, P. subsibiricus, P. tennesseensis, P. vaseyi, P. zosteriformis
P. alpinus, P. amplifolius, P. bicupulatus, P. clystocarpus, P. confervoides, P. crispus, P. epihydrus, P. floridanus, P. foliosus, P. friesii, P. gramineus, P. groenlandicus, P. hillii, P. illinoensis, P. natans, P. nodosus, P. oakesianus, P. oblongus, P. obtusifolius, P. ogdenii, P. perfoliatus, P. praelongus, P. pulcher, P. pusillus, P. richardsonii, P. robbinsii, P. spirillus, P. strictifolius, P. subsibiricus, P. tennesseensis, P. vaseyi, P. zosteriformis
Synonyms P. capillaceus
Name authority Oakes: Magazine of horticulture, botany and all useful discoveries and improvements in rural affairs 7: 180. (1841) Rafinesque: Medical Repository hexade 2, 5:354. (1808)
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