hy·dril·la 
(hī-drĭlə)
(hī-drĭlə)n.
An aquatic plant (Hydrilla verticillata) native to Eurasia, Africa, and Australia, having whorled, lance-shaped leaves and small white flowers. It is considered a weed in North America, where it forms large dense mats in waterways.
[New Latin, genus name, diminutive of Latin hydra, hydra; see HYDRA.]