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.
[1913 Webster]