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blunt-leaf pondweed, potamot à feuilles obtuses

Illinois pondweed, pondweed, potamot de i'Illinois, shining pondweed

Rhizomes

absent.

present.

Cauline stems

slightly compressed, without spots, 35–90 cm;

glands yellow-green to gold, 0.2–1 mm diam.

terete, without spots, 28–120 cm;

nodal glands absent.

Turions

terminal, abundant, 3.5–7.8 cm × 2.3–5.1 mm, soft;

leaves ± 2-ranked;

outer leaves 3–4 per side, base not corrugate, apex apiculate to obtuse;

inner leaves undifferentiated.

absent.

Leaves

submersed, ± spirally arranged, sessile, flaccid;

stipules persistent, inconspicuous, convolute, free from blade, white, not ligulate, 0.6–1.8 cm, fibrous, rarely shredding at tip, apex obtuse;

blade light green to somewhat reddish, linear, not arcuate, 3–8.2 cm × 1–3.5 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, obtuse or round-apiculate, lacunae in 1–3 rows each side of midrib;

veins 3.

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

Floating leaves

petioles continuous in color to apex, 2–9 cm;

blade adaxially light green, elliptic to oblong-elliptic, 4–19 cm × 20–65 mm, base cuneate, apex round-mucronate;

veins 13–29.

Submersed leaves

sessile or petiolate, lax;

stipules persistent, conspicuous, convolute, free from blade, light brown to red-brown, not ligulate, 1–8 cm, not fibrous, not shredding at tip, apex acuminate;

petioles if present 0.5–4 cm;

blade red-brown to light green, elliptic to lanceolate or rarely linear, often arcuate, 5–20 cm × 2–45 mm, base acute, margins entire, often crispate, apex not hoodlike, acute-mucronate, lacunae in 2–5 rows each side midrib;

veins 7–19.

Inflorescences

unbranched, emersed;

peduncles not dimorphic, axillary, erect, rarely recurved, cylindric, 0.8–1.9(–4.2) cm;

spike not dimorphic, cylindric, 8–13 mm.

emersed, unbranched;

peduncles not dimorphic, terminal or axillary, erect to ascending, cylindric, 4–30 cm;

spikes not dimorphic, cylindric, 25–70 mm.

Fruits

sessile, olive-green to brown, obovoid, turgid, abaxially keeled or not, laterally keeled or not, 2.5–3.6 × 1.7–2.4 mm, lateral keels when present without points;

beak erect, 0.8–1 mm;

sides without basal tubercles;

embryo with 1 full spiral.

sessile, grayish green to olive green, obovoid to ovoid, laterally compressed, abaxially keeled, laterally ridged, 2.5–3.6 × 2.1–3 mm, abaxial keel well developed, lateral ridges without points;

beak erect to slightly recurved, 0.5–0.8 mm;

sides without basal tubercles;

embryo with 1 full spiral.

2n

= 26.

= 104.

Potamogeton obtusifolius

Potamogeton illinoensis

Phenology Flowering and fruiting summer–fall. Flowering and fruiting summer–fall.
Habitat Medium- to low-alkaline waters of lakes and slow-flowing streams Alkaline waters of streams, rivers, lakes, ponds, and sloughs
Elevation 50–2000 m (200–6600 ft) 0–2700 m (0–8900 ft)
Distribution
from FNA
AK; CT; ID; MA; ME; MI; MN; MT; NH; NJ; NY; PA; RI; VT; WA; WI; WY; AB; BC; MB; NB; NB; NF; NS; NT; ON; PE; QC; SK; YT; Eurasia
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from FNA
AL; AR; CA; CO; CT; FL; GA; IA; IL; IN; KS; KY; MA; MD; MI; MN; MO; MT; NC; NE; NH; NJ; NY; OH; OK; OR; PA; SC; SD; TX; UT; VA; VT; WA; WI; WV; WY; BC; MB; NB; NT; ON; QC; Mexico; Central America; South America; West Indies
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Discussion

Potamogeton obtusifolius is a distinctive linear-leaved species with the leaf blades round at the apex, especially when fruiting inflorescences 5–7 mm wide are present. This is unusually wide for one of the linear-leaved species.

Two hybrids, Potamogeton obtusifolius × P. pusillus (= P. × saxonicus Hagström) and P. friesii × obtusifolius (= P. × semifructus A. Bennett ex Ascherson & Graebner), have been described.

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

Potamogeton illinoensis and P. gramineus are often difficult to separate. Certainly, in the extreme of each they are easily separated, but they continually grade into each other. Features to look for are the acute-mucronate apex of the submersed leaves of P. illinoensis and the acuminate apex for P. gramineus. Also, the number of veins seems to work as well.

Three hybrids, Potamogeton illinoensis × P. nodosus (= P. × faxonii Morong), P. amplifolius × P. illinoensis (= P. × scoliophyllus Hagström), and P. gramineus × P. illinoensis [= P. × spathuliformis (J. W. Robbins) Morong], have been described.

(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. 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
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. 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
Name authority Mertens & W. D. J. Koch: in J. C. Röhling et al., Deutschl. Fl., ed. 3 1: 855. (1823) Morong: Botanical Gazette 5: 50. (1880)
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