Potamogeton robbinsii
Potamogeton oakesianus
fern pondweed, potamot de Robbins, Robbin's pondweed
Oakes' pondweed, potamot d'Oakes
present.
present.
terete, without spots, to 100 cm;
glands absent.
terete, with red spots, 7–75 cm;
nodal glands absent.
absent.
absent.
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, ± spirally arranged.
petioles continuous in color to apex, 3.2–7.5 cm;
blade adaxially light to dark green, lanceolate to elliptic or ovate, (1.5–)2–4(–5.5) cm × 10–20(–30) mm, base rounded or tapering, apex acute;
veins (7–)9–19(–23).
sessile, lax;
stipules persistent, conspicuous, convolute, free from blade, whitish, not ligulate, 1–3 cm, delicately fibrous, not shredding at tip, apex obtuse;
blade pale green, linear to phyllodial, not arcuate, 5–16 cm × (0.25–)0.3–1 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex acute, not hoodlike, lacunae absent, setaceous;
veins 3.
often branched;
peduncles not dimorphic, axillary, erect, cylindric, 3–5(–7) cm;
spikes not dimorphic, moniliform (i.e., beaded), 7–20 mm.
unbranched, emersed;
peduncles not dimorphic, terminal, ascending to spreading, cylindric, 2.5–8 cm;
spikes not dimorphic, cylindric, 10–35 mm.
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, obovoid, turgid, abaxially keeled, laterally keeled, 2.5–3.5(–3.7) × (1.6–)2–2.4 mm, lateral keels without points;
beak erect, 0.4–0.8 mm;
sides without basal tubercles;
embryo with 1 full spiral.
apparently unknown.
= 52.
Potamogeton robbinsii
Potamogeton oakesianus
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 oakesianus, along with Potamogeton floridanus and P. natans, has floating leaves and phyllodial submersed leaves. The petioles of this species and of P. floridanus lack a short area of light-colored tissue immediately proximal to the blade.
(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
