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.
[1][2][3] . 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
red streaks. It reproduces primarily
vegetatively by fragmentation and by rhizomes and
turions (overwintering ), and flowers are rarely seen.
[2][4][5][6]
Hydrilla has a high resistance to
salinity (>9-10ppt) compared to many other freshwater associated aquatic plants.
The name Esthwaite Waterweed comes from its occurrence in
Esthwaite Water in northwestern England, the only English site where it is native, but now presumed extinct, having not been seen since 1941.
[7] Hydrilla closely resembles some other related aquatic plants, including
Egeria and
Elodea.
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