The Collaborative International Dictionary of English v.0.48:

Hygroplasm \Hy"gro*plasm\, n. [Gr. ? wet + ? form, mold.]
   (Biol.)
   The fluid portion of the cell protoplasm, in opposition to
   stereoplasm, the solid or insoluble portion. The latter is
   supposed to be partly nutritive and partly composed of
   idioplasm.
   [1913 Webster]