Source : Webster's Revised Unabridged Dictionary (1913)

Thallus \Thal"lus\, n.; pl. {Thalli}. [NL., fr. Gr. ? young
   shoot or branch, frond.] (Bot.)
   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.

Source : WordNet®

thallus
     n : a plant body without true stems or roots or leaves or
         vascular system; characteristic of the thallophytes
     [also: {thalli} (pl)]