Hydrilla (Esthwaite Waterweed or Hydrilla) is an aquatic plant
genus, usually treated as containing just one species, Hydrilla
verticillata, though some botanists divide it into several
species. Synonyms include H. asiatica, H. japonica, H.
lithuanica, and H. ovalifolica. It is native to the cool and
warm waters of the Old World in Asia, Europe, Africa and
Australia, with a sparse, scattered distribution; in Europe, it
is reported from Ireland, Great Britain, Germany, and the Baltic
States, and in Australia from Northern Territory, Queensland,
and New South Wales. . The stems grow up to 1–2 m long. The
leaves are arranged in whorls of two to eight around the stem,
each leaf 5–20 mm long and 0.7–2 mm broad, with serrations or
small spines along the leaf margins; the leaf midrib is often
reddish when fresh. It is monoecious (sometimes dioecious), with
male and female flowers produced separately on a single plant;
the flowers are small, with three sepals and three petals, the
petals 3–5 mm long, transparent with