The aquatic plant Hydrilla verticillata, aka Esthwaite waterweed, is easily confused
with waterweeds of the genera Egeria and Elodea. It is a variable species with many different-looking forms. Its main areas of distribution are the temperate to tropical climate zones of Asia and Australia, Eastern Africa and parts of Europe, however, it has been introduced in almost all regions of the world by human activity. In the USA, H. verticillata is an invasive, hard-to-control aquatic weed that outpopulates native aquatic plants.
The stems of H. verticillata can grow to a length of up to 2 metres and ramify moderately. Besides upright stems, the plant often also grows runner-like creeping shoots as well as subterranean sprouts with tuberous turions (winter buds) at their ends. In these, Esthwaite waterweed survives unfavourable periods inside the ground.
There are whorls of 3 to 9 leaves on each node. The leaf margin is serrate, which can clearly be seen with the naked eye (unlike Egeria densa and Elodea canadensis, whose leaf margin looks smooth to the naked eye and whose fine serration is only visible with a magnifying glass). On the older stem areas, the internodes are longer than the leaves as a rule (in Egeria they are shorter than the leaves).
To be continued...
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