1911 Encyclopædia Britannica/Water-thyme
WATER-THYME, known botanically as Elodea canadensis, a small submerged water-weed, native of North America. It was introduced into Co. Down, Ireland, about 1836, and appeared in England in 1841, spreading through the country in ponds, ditches and streams, which were often choked with its rank growth. Elodea is a member of the monocotyledonous natural order Hydrocharideae (q.v.).