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

672 HYDROCHARITACEAE/EGERIA
touch); male flowers borne at the water surface on a thread-like stalk, with conspicuous
white perianth parts 8–10 mm long _______________________________________________ Egeria
5. Leaves usually 1.5(–2) cm or less long, visibly serrate marginally and with teeth on the
midvein beneath (fresh leaves thus rough to the touch); male flowers detaching and
floating to the surface, the perianth parts translucent, colorless, 3–5 mm long _____________ Hydrilla
EGERIA Planch.
SOUTH AMERICAN ELODEA, BRAZILIAN WATERWEED
AA genus of 2 species of subtropical South America (Brazil and Argentina) (Haynes & Holm-
Nielsen 2001), sometimes treated in Elodea (an American genus of 5 species—Haynes 2000b).
Elodea canadensis Michx., known from OK but apparently not TX, can be distinguished from
the somewhat similar Egeria densa and Hydrilla verticillata using the following characters:
leaves opposite or in whorls of three, mostly 0.8–1.5 cm long, leaf margins lacking teeth perceptible
to the naked eye, and leaf midveins lacking teeth. Egeria has nectariferous flowers and a
relatively large perianth (Catling & Wojtas 1986); it is presumably animal-pollinated, in contrast
to the many hydrophilous taxa observed in this family. (Latin, egeri, a nymph, named after
a Roman goddess of water or mythical water nymph, in reference to the aquatic habitat)
REFERENCES: St. John 1965; Lazor 1975; Cook & Urmi-König 1984; Catling & Wojtas 1986.
Egeria densa Planch., (dense), WATERWEED, BRAZILIAN WATERWEED, COMMON WATERWEED. Perennial,
submersed aquatic, rooting at bottom or drifting if broken; lowermost leaves opposite or in
whorls of 3; rest of leaves in whorls of 4–6(–8); leaves crowded, usually linear-lanceolate, serrulate,
transparent, usually 2–3(–4) cm long and 3(–5) mm wide; flowers 3-merous; male spathes
2–4-flowered, in upper axils; male flowers reaching the surface on a long thread-like hypanthium
3–6 cm long; petals white, ca. 8–10 mm long, much longer than the sepals; anthers 9;
female flowers not observed (female plants not observed outside the species’ native range, and
reproduction here thus entirely by vegetative means—Haynes & Holm-Nielsen 2001). Lakes
and ponds, probably rapidly spreading; Dallas, Grayson (Lake Ray Roberts), Hays, Lamar (Pat
Mayse Lake) (BRIT), Harrison, Marion, Nacogdoches, Sabine, San Augustine (ASTC), Leon, and
Newton (Turner et al. 2003) cos.; also Brown and Comanche (HPC) cos. in Cross Timbers and
Prairies; sw Canada (B.C.) and widespread in the U.S. except the nc part. [Anacharis densa
(Planch.) Vict., Elodea densa (Planch.) Casp.] Native of se Brazil. This species is widely sold for
use in aquaria but is probably spread from lake to lake mainly by boats or boat trailers. It is also
widely used in biological laboratories because of the thin (2 cell-layers thick) leaves which can
be easily used to demonstrate plant cell structure and cytoplasmic streaming (Haynes 2000b).
It is commonly called “elodea” even though this is the scientific name of a related genus. This
species is considered noxious in CA, NC, OR, and WA (Kartesz 1999). � I
Egeria najas Planch., (for Najas, naiad, water-nymph, bushy-pondweed, another genus in the
Hydrocharitaceae), NARROW-LEAF ANACHARIS, LIGHT GREEN ANACHARIS, the other member of
the genus and also a native of southeastern Brazil, is widely used in the aquarium trade and is
of potential concern as a problematic exotic (Catling & Mitrow 2001). While not known from
TX, it is mentioned here as a note to alert collectors of it as a possibility. It resembles both Egeria
densa and Hydrilla verticillata but can be distinguished by the following key from Catling
and Mitrow (2001).
1. Leaves with obscure serrations or marginal prickles (requiring magnification to be readily observed)
and leaf edges straight _________________________________________________________ E. densa
1. Leaves with prominent serrations or marginal prickles (readily seen without magnification) and
leaf edges conspicuously concave or straight between the serrations.
2. Leaves long-attenuate, concave between the serrations, their midvein without prickles on
abaxial (lower) leaf surface; scales in the leaf axis absent or to 0.36 mm long, mostly smooth-