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

1110 POTAMOGETONACEAE
petiolate, mostly basal; petiolate leaves with petioles with long-clasping basal portion; leaf
blades shorter than the petioles, 6–22 cm long, narrowly ovate to triangular-lanceolate, the base
rounded-truncate to deeply cordate; inflorescence a slender terminal spike from a spathe; perianth
violet-blue, the upper lobe with a central 2-lobed yellow-green or yellow spot; fruit a 1seeded
utricle. Margins of lakes, ponds, and streams; widespread in East TX; also Comanche Co.
(Turner et al. 2003) in the Cross Timbers and Prairies and n Gulf Prairies and Marshes; throughout
the e U.S. w to MI, KS, OK, and TX, also se Canada. Jun–Oct. [P. cordata var. lanceolata (Nutt.)
Griseb., P. cordata var. lancifolia (Muhl. ex Elliott) Torr.] This species is sometimes cultivated as
an ornamental; the fruit is said to be edible (Mabberley 1997). Tristyly (= pollination system involving
styles of three lengths) is known in this species (Ornduff 1966; Price & Barrett 1982,
1984), and bumble bees are the most abundant pollinators (Wolfe & Barrett 1989). m/296
POTAMOGETONACEAE Dumort.
PONDWEED FAMILY
Aquatic rhizomatous herbs; leaves in ours alternate or closely crowded, all submersed or with
both submersed and floating blades, sessile or petiolate; flowers perfect, in 2–5 whorls on pedunculate
axillary spikes; perianth 4-merous, inconspicuous; stamens 4; carpels 4, free, sessile;
ovaries superior; fruits drupe-like, 1-seeded.
AA small (ca. 90–100 species in 3 genera—Haynes & Hellquist 2000b; Haynes & Holm-
Nielsen 2003) cosmopolitan family of perennial, rooted aquatics. While pollination in most
members of the family is thought to be by anemophily (= wind-pollination) (Haynes et al.
1998c; Haynes & Holm-Nielsen 2003), hypohydrophily (= pollination under water) is known
for Potamogeton pusillus var. tenuissimus (Philbrick 1983[1984]; Haynes et al. 1998c), and hydroautogamy
(= self-pollination underwater) is thought to occur in a number of species (Philbrick
& Anderson 1987). Haynes (1974) and Haynes and Hellquist (2000b) indicated that, like other
aquatic vascular plants, members of the Potamogetonaceae are known for their phenotypic
plasticity. However, individuals in fruit “have relatively consistent morphology within a species”
(Haynes & Hellquist 2000b). Thus, for positive identification, it is essential to have specimens
in reproductive condition. Vegetative reproduction by stem fragmentation or by turions
(= swollen structure or offshoot, often serving to overwinter) is well known in the family. The
third genus in the family (not found in East TX) is the monotypic Old World Groenlandia
(with opposite leaves). Earlier molecular analyses (Duvall et al. 1993b; Les et al. 1993) indicated a
relationship between Potamogetonaceae and Alismataceae or Zosteraceae (EEL-GRASS FAMILY),
while more recent molecular results (Les & Haynes 1995) suggest a closer link to
Zannichelliaceae and then Zosteraceae. In fact, Potamogetonaceae and Zannichelliaceae appear
to be closely related sister groups (Haynes et al. 1998d), and some authorities (e.g., APG 1998;
APG II 2003) now submerge the Zannichelliaceae into the Potamogetonaceae. (subclass
Alismatidae—Cronquist; order Alismatales—APG II)
FAMILY RECOGNITION IN THE FIELD: aquatics with leaves all submersed or often with both submersed
and floating leaves—the floating ones � elliptic and rather leathery with a waxy, waterrepellent
upper surface; flowers small, inconspicuous, 4-merous, in dense, pedunculate, axillary
spikes, the spikes not subtended by bracts.
REFERENCES: Morong 1893; Taylor 1909a; Haynes 1978; Dahlgren et al. 1985; Larsen & Barker
1986; Haynes et al. 1998c; Haynes & Hellquist 2000b; Haynes & Holm-Nielsen 2003.
1. Floating leaves present or absent; stipules free of the leaf base or adnate for � 4(–10) mm, often
early deteriorating; submersed leaves translucent, flat, without grooves or channels, of variable
width, sometimes less but often much wider than 1 mm (to 45 mm wide); peduncle stiff, if long
enough the inflorescence thus held above the water surface; including species widespread and
common in East TX ____________________________________________________________ Potamogeton