What is "hippuris vulgaris"

Mare's-tail \Mare's"-tail`\, n.

1. A long streaky cloud, spreading out like a horse's tail, and believed to indicate rain; a cirrus cloud. See Cloud.

   Mackerel sky and mare's-tails Make tall ships carry low sails.
   --Old Rhyme.

2. (Bot.) An aquatic plant of the genus Hippuris ( Hippuris vulgaris), having narrow leaves in whorls.

Hippuris vulgaris (from Greek: ἵππος — horse and οὐρά — tail), known as mare's-tail or common mare's tail, is a common aquatic plant of Eurasia and North America ranging from Greenland to the Tibetan Plateau to Arizona. It prefers non-acidic waters. The species is also sometimes called horsetail, a name which is better reserved to the horsetails of genus Equisetum. These are unrelated to H. vulgaris, although there is some resemblance in appearance.