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

HYDRILLIA/HYDROCHARITACEAE 673
margined and lacking a fringe of orange-brown hairs; internodes mostly much shorter than
the leaves and up to 10 mm long 1 dm below the growing tip; flowers with round petals and
with nectaries _____________________________________________________________________ E. najas
2. Leaves linear to attenuate, straight to slightly concave between the serrations, their midvein
sometimes with prickles on the abaxial leaf surface; scales in the leaf axis mostly ca. 0.5 mm
long, mostly with a fringe of orange-brown hairs; internodes mostly as long as the leaves and
up to 50 mm long 1 dm below the growing tip; flowers with linear petals, lacking nectaries
________________________________________________________________________ Hydrilla verticillata
HYDRILLA Rich. WATER-THYME
AA monotypic Old World genus (Haynes & Holm-Nielsen 2001) introduced into the U.S. and
Central America. The male flowers are released underwater, rise unopened to the surface, and
float to the female flowers (an example of epihydrophily—see discussion in Vallisneria generic
synopsis). Cook and Lüönd (1982) indicated that during flower opening the stamens spring
upward, the anthers burst, and pollen is scattered in the air. Some of this lands on the female
flowers and some on the surface of the water; apparently the pollen grains landing on the water
are lost in terms of reproduction and only those actually landing on the female flower are involved
in pollination. Even though the effective pollen grains never contact the water, the male
flowers float and the system can thus be described as epihydrophily. (Greek: hydr-, water, in
reference to the habitat, plus the diminutive suffix—Haynes & Holm-Nielsen 2001)
REFERENCES: Lazor 1975; Godfrey & Wooten 1979; Cook & Lüönd 1982; Pieterse 1983; Steward et
al. 1984; Langeland 1996.
Hydrilla verticillata (L.f.) Royle, (whorled), WATER-THYME. Perennial, submersed, dioecious or
monoecious aquatic; horizontal stems in the substrate sometimes forming turions (= swollen
structure or offshoot, often serving to overwinter); erect stems branching, capable of ascending
as much as 8.5 m (Godfrey & Wooten 1979) and growing horizontally near the water surface;
leaves in whorls of (2–)3–8, sessile, mostly 1.5(–2) cm or less long, 1.5–2 mm wide, narrowly oblong,
visibly serrate marginally, with a single vein which on the lower surface bears conical
protrusions tipped with teeth (fresh leaves thus noticeably rough to the touch); staminate flowers
detaching and floating to the surface; pistillate flowers reaching the surface at the end of an
elongate, thread-like floral tube 4–5 cm long; perianth segments 6, colorless, very inconspicuous,
3–5 mm long; fruits 5–6 mm long, � fusiform. Lakes and other aquatic habitats; rapidly
spreading at present in East TX (M. Smart, pers. comm); probably spread vegetatively from lake
to lake by boats or boat trailers and also intentionally by fishermen (L. Hartman, pers. comm.)
to “improve” the habitat (this is both illegal and ill-advised, since it ultimately degrades the
fishery); scattered in e 1/3 of TX. Summer–fall. Native of the Old World, probably the warmer
areas of Asia (Cook & Lüönd 1982). Austin (1978) indicated that the species apparently first
entered the U.S. at Miami, FL in 1959, while Blackburn et al. (in Steward et al. 1984) noted that
female plants were first reported in the U.S. in 1960 from South Florida (misidentified as Elodea
canadensis). Monoecious plants were first observed in the U.S. in Washington, D.C. in 1982. Female
(dioecious) individuals are triploid, while monoecious plants are diploid (Steward et al.
1984). Since its introduction in FL, this problematic species has spread across the southern U.S.
to TX, AZ, CA, and WA, and up the east coast to DE, MD, and CT. The earliest TX collection we
are aware of is from 1974 (Amerson 2097, BRIT), while the species was first collected in LA in
1973 (Solymosy 1974). Hydrilla verticillata is a serious invasive pest which can completely
dominate aquatic habitats, eliminating native species, clogging waterways, and severely curtailing
recreational use (Steward et al. 1984; Flack & Furlow 1996; Langeland 1996; Kartesz
1999). It is considered by many wetland biologists as “potentially the most dreaded species of
aquatic weed in warm temperate areas” (Crow & Hellquist 2000a). In TX, H. verticillata is considered
a “harmful or potentially harmful exotic plant,” and it is illegal to release, import, sell,