The Collaborative International Dictionary of English v.0.48:

Hydrorhiza \Hy`dro*rhi"za\, n.; pl. L. Hydrorhiz[ae], E.
   Hydrorhizas. [NL., fr. E. hydra + Gr. ? a root.] (Zool.)
   The rootstock or decumbent stem by which a hydroid is
   attached to other objects. See Illust. under Hydroidea.
   [1913 Webster]