Hydrilla (Esthwaite Waterweed, waterthyme or hydrilla) is a genus of aquatic plant that is usually treated as containing only one species:
Hydrilla Verticillata. Although some botanists divide this category into several species. The plant 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 the Northern Territory, Queensland and New South Wales.
The plant is sometimes invasive and unwanted. For example, in Galveston Bay and parts of Florida this plant is considered an invasive species.
The stems grow up to 1-2m long. The leaves are arranged in whorls of two to eight around the stem, each leaf 5-20mm long and 0.7-2mm broad with serrations or small spines along the leaf margins. The leaf midrib is often reddish in color when fresh. The plant is monoecious (sometimes dioecious), with male and female flowers produced separately on a single plant. The flowers are small, with three sepas and three petals. The petals are 3-5mm long with transparent and red streaks on them. Hydrilla verticillata reproduces primarily vegetative by fragmentation and by rhizomes and turions (overwintering) and the flowers are rarely seen.
Air spaces keep the plant upright. Hydrilla has a high resistance to salinity 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 the plant is native, but not presumed extinct as it has not been seen since 1941.
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| Hydrilla Verticillata captured under the microscope at 40x. |
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| Hydrilla Verticillata captured under the microscope at 100x. |
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| Hydrilla Verticillata captured under the microscope at 400x. |
