keys to the vascular plants of east texas - Botanical Research ...

POTAMOGETON/POTAMOGETONACEAE 1113
Potamogeton foliosus Raf., (leafy), LEAFY PONDWEED. Nodal glands usually absent, only rarely
present; leaves all submersed, sessile, linear, 1.3–8.2 cm long, 0.3–2.3 mm wide; turions uncommon.
Walker and Williamson (Ogden 1966) cos. in the Post Oak Savannah and s Blackland
Prairie (no recent records for East TX); the range map in Haynes and Hellquist (2000b) indicated
that P. foliosus is of questionable occurrence for East TX, and we have seen no confirming
specimens; sparsely scattered in TX, mainly in the w part of the state; widespread in s Canada
and throughout most of the U.S. May–Oct. [P. foliosus var. macellus Fernald] Among the linearleaved
Potamogeton species, P. foliosus is distinctive in having fruits with an undulating winglike
dorsal (abaxial) keel (Haynes & Hellquist 2000b). This species is considered to be related to
P. pusillus (Wiegleb & Kaplan 1998).
Potamogeton illinoensis Morong, (of Illinois), SHINING PONDWEED, CORNSTALK PONDWEED,
ILLINOIS PONDWEED. Submersed leaves sessile or tapering to a petiole to 4 cm long, with blades
5–20 cm long, 10–45 mm wide, elliptic to oblong-elliptic, linear-lanceolate, or lanceolate; floating
leaves sometimes absent, if present, the blades 4–8(–19) cm long, 1–4(–7) cm wide; turions
absent. Hays, Travis (BRIT), Austin, Bell, Bexar, Williamson (Ogden 1966), Comal, and Grayson
(Turner et al. 2003) cos.; widely scattered in TX, but mainly Edwards Plateau; s Canada and
throughout much of the U.S. Apr–Jun.
Potamogeton nodosus Poir., (knotty), LONG-LEAF PONDWEED. Submersed leaves often disintegrated
by flowering time or sometimes persistent, tapering to a petiole, the blades 10–20(–30)
cm long, 10–20(–35) mm wide, linear-lanceolate to lanceolate-elliptic; floating leaves with
blades 4–10(–13) cm long, (1.5–)2–3(–4.5) cm wide; turions absent. Nearly throughout TX; s
Canada and throughout most of the U.S. Apr–Jun. This is the most common PONDWEED in
much of East TX. It is widespread geographically (both New and Old worlds) and is reported to
be “one of the most polymorphic Potamogeton species” (Wiegleb & Kaplan 1998).
Potamogeton pulcher Tuck., (handsome), HEART-LEAF PONDWEED, SPOTTED PONDWEED. Stems
usually conspicuously black-spotted; submersed leaves sessile or tapering to a short petiole, the
blades oblong to lanceolate or linear-lanceolate, to 18 cm long and 35 mm wide (usually
smaller), usually persistent, only rarely disintegrating by flowering time; floating leaves with
blades 4.5–9(–11) cm long, 2–5.5(–8.5) cm wide; turions absent. Pineywoods and n Post Oak Savannah;
se Canada (N.S.) and nearly throughout the e U.S. w to MN and TX. Apr–May. A similar
species, P. amplifolius, that is expected for East TX, is discussed in a note below.
Potamogeton pusillus L., (very small). Nodal oil glands present on at least some nodes; leaves all
submersed, sessile, linear, 0.9–6.5 cm long, 0.2–2.5 mm wide; turions common. Late May–Jun.
Some authorities (e.g., Godfrey & Wooten 1979) do not recognize infraspecific taxa; Haynes
(1974) indicated that while recognition at the varietal level is most appropriate, there is considerable
intergradation and many individuals cannot be identified to variety with certainty.
More recently, Haynes and Hellquist (1996) have recognized the variation at the subspecies
level. The following key to subspecies is modified from Haynes (1974) and Haynes and
Hellquist (2000b):
1. Mature fruits widest above the middle, the sides concave; beak of fruit toward adaxial edge,
rarely median; peduncles filiform to cylindric, 1–3 per plant; inflorescences interrupted, usually
of 2–4 distinct verticils (= whorls); leaf blades with 0–2 rows of lacunae along midrib, apically
acute, rarely apiculate, rarely with a bristle; stipules connate; subspecies widespread in East TX
____________________________________________________________________________ subsp. pusillus
1. Mature fruits mostly widest at or below the middle, the sides rounded; beak of fruit median, not
toward adaxial edge; peduncles cylindric, more than 3 per plant; inflorescences continuous, mostly
of 1–2(–3) adjacent verticils; leaf blades with 1–5 rows of lacunae along midrib, apically acute to
obtuse; stipules convolute, mostly not connate; subspecies known in East TX only from extreme
ne part of area ____________________________________________________________ subsp. tenuissimus