Hymenachne amplexicaulis, also known as West Indian marsh grass in the US, Olive hymenachne in Australia, and hereafter referred to as hymenachne, is New World species in the genus Hymenachne. Hymenachne is from the Greek hymen meaning "membrane" and achne meaning "chaff, glume, scale" and amplexicaulis is from Latin, literally "embracing the stem" or "stem-clasping. This freshwater species is native to the tropical and subtropical regions of the West Indies, Northern South America, and Central America.

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