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small pondweed

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

Rhizomes

absent.

present.

Cauline stems

terete to slightly compressed, without spots, 18–150 cm;

glands present on at least some nodes, green, gold, brown, or rarely white, to 0.5 mm diam.

terete, without spots, 28–120 cm;

nodal glands absent.

Turions

common, soft, lateral or terminal, 0.9–3.2 cm × 0.3–1.8 mm, soft;

leaves ± 2-ranked;

outer leaves 1–3 per side, base not corrugate, apex subulate to obtuse;

inner leaves rolled into hardened fusiform structure.

absent.

Leaves

submersed, ± spirally arranged, sessile, delicate to coarse;

stipules persistent, inconspicuous, connate or convolute, free from base of blade, brown to green or white, not ligulate, 0.31–0.92 cm, rarely appearing fibrous, not shredding at tip, apex obtuse;

blade pale green to olive-green, rarely somewhat reddish, linear, not arcuate, 0.9–6.5 cm × 0.2–2.5 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, subulate to obtuse, lacunae absent or present, in 0–5 rows each side of midrib;

veins 1–3(–5).

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, submersed or emersed;

peduncles not dimorphic, axillary or terminal, erect, rarely recurved, filiform to slightly clavate, 0.5–6.2(–6.6) cm;

spikes not dimorphic, capitate to cylindric, 1.5–10.1 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, green to brown, ovoid to obovoid, turgid to concave, not abaxially or laterally keeled, 1.5–2.2 × 1.2–1.6 mm;

beak erect, 0.1–0.6;

sides without basal tubercles;

embryo with less than 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

= 104.

Potamogeton pusillus

Potamogeton illinoensis

Phenology Flowering and fruiting summer–fall.
Habitat Alkaline waters of streams, rivers, lakes, ponds, and sloughs
Elevation 0–2700 m [0–8900 ft]
Distribution
from FNA
AK; AL; AR; AZ; CA; CO; CT; DC; DE; FL; GA; IA; ID; IL; IN; KS; KY; LA; MA; MD; ME; MI; MN; MO; MS; MT; NC; ND; NE; NH; NJ; NM; NV; NY; OH; OK; OR; PA; RI; SC; SD; TN; TX; UT; VA; VT; WA; WI; WV; WY; AB; BC; MB; NB; NS; NT; NU; ON; PE; QC; SK; YT; nearly worldwide
[WildflowerSearch map]
[BONAP county map]
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
[WildflowerSearch map]
[BONAP county map]
Discussion

Three hybrids, Potamogeton perfoliatus × P. pusillus (= P. × mysticus Morong), P. friesii × P. pusillus (= P. × pusilliformis Fisher [P. × intermedius Fischer]), and P. obtusifolius × P. pusillus (= P. × saxonicus Hagström), have been described.

Subspecies 3 (3 in the flora).

(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.)

Key
1. Leaf blade 1-veined, subulate, 0.2–0.7 mm wide; New England and s Quebec.
subsp. gemmiparus
1. Leaf blade 1–5-veined, acute to obtuse, 0.2–2.5 mm wide; widespread throughout North America.
→ 2
2. Mature fruits obovoid, sides concave; beak toward adaxial edge, rarely median; peduncles filiform to cylindric, 1–3 per plant; inflorescences interrupted; leaf blade with 0–2 rows of lacunae along midrib, apex acute, rarely apiculate, rarely with bristle; stipules connate.
subsp. pusillus
2. Mature fruits widest at middle or ovoid, sides rounded; beak median, not toward adaxial edge; peduncles cylindric, more than 3 per plant; inflorescences continuous; leaf blade with 1–5 rows of lacunae along midrib, apex acute to obtuse; stipules convolute.
subsp. tenuissimus
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. 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
Subordinate taxa
P. pusillus subsp. gemmiparus, P. pusillus subsp. pusillus, P. pusillus subsp. tenuissimus
Name authority Linnaeus: Sp. Pl. 1: 127. 1753 (as pusillum) Morong: Botanical Gazette 5: 50. (1880)
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