thallus > define
noun
- A plant body without true stems or roots or leaves or vascular system; characteristic of the thallophytes. .
by WordNet 3.0 © 2006 by Princeton University
(n.) A solid mass of cellular tissue, consisting of one or more layers, usually in the form of a flat stratum or expansion, but sometimes erect or pendulous, and elongated and branching, and forming the substance of the thallogens.
by Webster's Revised Unabridged Dictionary