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

POTAMOGETON/POTAMOGETONACEAE
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1. Floating leaves absent (all leaves submersed); stipules adnate (= fused) to the leaf base for a
distance of 10–30 mm, the free portion projecting as a ligule � half as long as the adnate portion;
submersed leaves opaque, turgid, channeled, 0.2–1 mm wide; peduncle flexible, not holding
the inflorescence above the water surface; species rare in East TX ________________________ Stuckenia
POTAMOGETON L. PONDWEED
Plants glabrous perennials; nodes sometimes with oil glands; plants with both submersed (usually
filiform to � lanceolate, thin and flexuous) and floating (usually � elliptic and rather
leathery, with a waxy upper surface) leaf blades or all leaves submersed; leaves sessile or petiolate;
flowers in pedunculate axillary spikes, these usually held above the water.
AA cosmopolitan genus of ca. 95 species (Haynes & Holm-Nielsen 2003), with the highest
density of species in e North America (Wiegleb 1988). Wiegleb and Kaplan (1998), however,
considered the genus to have ca. 69 species. It is renowned for its extensive morphological diversity,
including species which can be characterized as heterophyllous (with both floating and
submersed leaves) or homophyllous (with only submersed leaves) (Wiegleb 1988; Les & Sheridan
1990a, 1990b). PONDWEEDS are very important as a source of food for waterfowl (e.g., ducks)
and as food and habitat for aquatic animals (Haynes 1974). In fact, in terms of its effect on animal
life, it is “one of the most important genera in the aquatic environment” (Haynes & Hellquist
2000b). Because Potamogeton species are aquatic and difficult to collect, and because identification
can be problematic, the group is neither well-collected nor well known. They are found in
lakes, ponds, streams, or other aquatic habitats. Some species of Potamogeton exhibit
hydrophily or water-mediated pollination (Cox 1988; Philbrick 1988). The underwater leaves of
heterophyllous species and leaves (all underwater) of homophyllous species have reduced levels
of flavonoids, while floating leaves of heterophyllous species have glycoflavones (Les &
Sheridan 1990b); according to Les and Sheridan (1990b) the glycoflavones “are probably maintained
in floating leaves because of their UV [radiation] filtering ability. … The lack of
glycoflavones in submersed leaves … is attributable to the ability of naturally colored water to
significantly absorb harmful UV radiation.” Potamogeton species with submersed linear leaves
are sometimes confused with two other linear-leaved aquatic genera, Najas (Hydrocharitaceae)
and Zannichellia (Zannichelliaceae); however, both of these other genera have mostly opposite
or whorled leaves, versus leaves alternate in Potamogeton (Yatskievych 1999). (Ancient name
from Greek: potamos, river, and geiton, a neighbor, from the aquatic habitat)
REFERENCES: St. John 1916; Fernald 1932; Ogden 1943, 1966; Klekowski & Beal 1965; Haynes 1968,
1974, 1986; Reznicek & Bobbette 1976; Stuckey 1979; Les 1983; Philbrick 1983[1984]; Catling &
Dobson 1985; Philbrick & Anderson 1987; Wiegleb 1988; Les & Sheridan 1990a, 1990b; Haynes
& Hellquist 1996; Hollingsworth et al. 1998; Wiegleb & Kaplan 1998.
1. Submersed leaves 2.5 mm or less wide, mostly 20 times or more longer than wide, linear; floating
leaves, if present, with blades ≤ 40 mm long (usually much less).
2. Floating petiolate leaves with blades 5–40 mm long usually present; stipules usually adnate
to the base of the submersed sessile leaf blades for ca. 1–4 mm, forming a sheath partially
clasping the stem—leaf blades therefore apparently arising � from the top of the sheath
(however, note that stipule tips are free); fruit wall papery thin, the embryo coil plainly visible
within _____________________________________________________________________ P. diversifolius
2. Floating petiolate leaves absent, the leaves all alike, sessile, submersed; stipules free of the
submersed sessile leaf blades; fruit wall firm, obscuring the embryo coil.
3. Fruits dorsally smooth and rounded; stipules with veins usually not very evident; nodes of
the stem usually with a pair of minute oil glands; species widespread in East TX ____________ P. pusillus
3. Fruits with an undulate to dentate, dorsal ridge or keel; stipules with evident veins appearing
as ridges extending the length of the stipules; nodes of the stem usually without a pair
of minute glands; species of questionable occurrence in East TX ________________________ P. foliosus