Burke: Potamogeton robbinsii vs. Potamogeton pusillus

	Potamogeton robbinsii	Potamogeton pusillus
	fern pondweed, Robbin's pondweed	small pondweed
Habit	Aquatic, herbaceous perennials from slender rhizomes, the stems terete, stout, branching, 1-4 dm. long.	Aquatic, herbaceous perennials from slender rhizomes, the stems very slender, freely branched, up to 10 dm. long.
Leaves	Leaves alternate, submersed, linear-lanceolate, up to 12 cm. long, mostly 3-4 mm. broad, acute, finely serrulate, with a broad mid-nerve and 20-35 very fine lateral nerves, stiffly 2-ranked and crowded on the stem; stipules fused to the leaf bases for 10-15 mm., the free portion longer, white.	Leaves alternate, submersed, narrowly linear, up to 7 cm. long and 0.5-2 mm. broad, 3-nerved, acute, bordered at the base with small, globose glands; stipules 6-15 mm. long, surrounding the stem for half their length.
Flowers	Stem freely-branching in the inflorescence, with many peduncles 3-5 cm. long; flowers perfect, in spikes 7-20 mm. long on the peduncles; perianth of 4 clawed segments, each with an upturned, oval blade; stamens 4, fused with the claws; pistils 4, sessile.	Flowers perfect, in spikes 3-15 cm. long but with only 1-3 whorls, on very slender peduncles 1.5-4 cm. long; perianth of 4 clawed segments, each with an upturned, oval blade; stamens 4, fused with the claws; pistils 4, sessile.
Fruits	Achenes obovoid, 3.5-4 mm. long, with a sharp dorsal keel and rounded lateral keels, and without a beak.	Achenes obliquely obovoid, about 2 mm. long, not keeled, the stylar beak 0.5 mm. long, slightly recurved.

	Potamogeton robbinsii	Potamogeton pusillus
Flowering time	August - September	July - August
Habitat	Quiet water, mostly in lakes.	Shallow ponds and streams.
Distribution	Occurring on both sides of the Cascades crest in Washington; Alaska to California, east to Montana and Colorado; Minnesota to Maine, south to Virginia in eastern United States. [WildflowerSearch map] [BONAP county map]	British Columbia south on both sides of the Cascades to California, east to the Atlantic. [WildflowerSearch map] [BONAP county map]
Origin	Native	Native
Conservation status	Not of concern	Not of concern
Sibling taxa	P. alpinus, P. amplifolius, P. berchtoldii, P. crispus, P. epihydrus, P. fibrillosus, P. foliosus, P. friesii, P. gramineus, P. illinoensis, P. natans, P. nodosus, P. obtusifolius, P. praelongus, P. pusillus, P. richardsonii, P. strictifolius, P. zosteriformis	P. alpinus, P. amplifolius, P. berchtoldii, P. crispus, P. epihydrus, P. fibrillosus, P. foliosus, P. friesii, P. gramineus, P. illinoensis, P. natans, P. nodosus, P. obtusifolius, P. praelongus, P. richardsonii, P. robbinsii, P. strictifolius, P. zosteriformis
Web links	Local floras: BC, CA, OR, WA Local Web sites: CalFlora, CalPhotos, Flora NW, PNW Herbaria WildflowerSearch iNaturalist (observations) USDA Plants Database LBJ Wildflower Center SEINet Plants of the World Online Encyclopedia of Life Wikipedia Google Image Search	Local floras: BC, CA, OR, WA Local Web sites: CalFlora, CalPhotos, Flora NW, PNW Herbaria WildflowerSearch iNaturalist (observations) USDA Plants Database LBJ Wildflower Center SEINet Plants of the World Online Encyclopedia of Life Wikipedia Google Image Search

Potamogeton pusillus

fern pondweed, Robbin's pondweed

small pondweed

Habit

Aquatic, herbaceous perennials from slender rhizomes, the stems terete, stout, branching, 1-4 dm. long.

Aquatic, herbaceous perennials from slender rhizomes, the stems very slender, freely branched, up to 10 dm. long.

Leaves

Leaves alternate, submersed, linear-lanceolate, up to 12 cm. long, mostly 3-4 mm. broad, acute, finely serrulate, with a broad mid-nerve and 20-35 very fine lateral nerves, stiffly 2-ranked and crowded on the stem;

stipules fused to the leaf bases for 10-15 mm., the free portion longer, white.

Leaves alternate, submersed, narrowly linear, up to 7 cm. long and 0.5-2 mm. broad, 3-nerved, acute, bordered at the base with small, globose glands;

stipules 6-15 mm. long, surrounding the stem for half their length.

Flowers

Stem freely-branching in the inflorescence, with many peduncles 3-5 cm. long;

flowers perfect, in spikes 7-20 mm. long on the peduncles;

perianth of 4 clawed segments, each with an upturned, oval blade;

stamens 4, fused with the claws;

pistils 4, sessile.

Flowers perfect, in spikes 3-15 cm. long but with only 1-3 whorls, on very slender peduncles 1.5-4 cm. long;

perianth of 4 clawed segments, each with an upturned, oval blade;

stamens 4, fused with the claws;

pistils 4, sessile.

Fruits

Achenes obovoid, 3.5-4 mm. long, with a sharp dorsal keel and rounded lateral keels, and without a beak.

Achenes obliquely obovoid, about 2 mm. long, not keeled, the stylar beak 0.5 mm. long, slightly recurved.

Potamogeton robbinsii

Potamogeton pusillus

Flowering time

August - September

July - August

Habitat

Quiet water, mostly in lakes.

Shallow ponds and streams.

Distribution

Occurring on both sides of the Cascades crest in Washington; Alaska to California, east to Montana and Colorado; Minnesota to Maine, south to Virginia in eastern United States.

[WildflowerSearch map]

[BONAP county map]