Potamogeton obtusifolius
Potamogeton robbinsii
blunt-leaf pondweed, potamot à feuilles obtuses
fern pondweed, potamot de Robbins, Robbin's pondweed
absent.
present.
slightly compressed, without spots, 35–90 cm;
glands yellow-green to gold, 0.2–1 mm diam.
terete, without spots, to 100 cm;
glands absent.
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.
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.
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.
unbranched, emersed;
peduncles not dimorphic, axillary, erect, rarely recurved, cylindric, 0.8–1.9(–4.2) cm;
spike not dimorphic, cylindric, 8–13 mm.
often branched;
peduncles not dimorphic, axillary, erect, cylindric, 3–5(–7) cm;
spikes not dimorphic, moniliform (i.e., beaded), 7–20 mm.
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.
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.
= 26.
= 52.
Potamogeton obtusifolius
Potamogeton robbinsii
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 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.)
- Local floras:
BC, 
CA, 
OR, 
WA - Local Web sites: CalFlora, CalPhotos,
Flora NW, 
Go Botany, 
IL Wildflowers, 
MI Flora, 
MN Wildflowers, 
PNW Herbaria 
WildflowerSearch
iNaturalist (observations)
USDA Plants Database- LBJ Wildflower Center
- SEINet
- Plants of the World Online
- Encyclopedia of Life
- Wikipedia
- Google Image Search
