The Collaborative International Dictionary
Hydrorhizae
Hydrorhiza \Hy`dro*rhi"za\, n.; pl. L. Hydrorhiz[ae], E. Hydrorhizas. [NL., fr. E. hydra + Gr. ? a root.] (Zo["o]l.) The rootstock or decumbent stem by which a hydroid is attached to other objects. See Illust. under Hydroidea.
Wiktionary
hydrorhizae
n. (plural of hydrorhiza English)