Thallus n. (pl. thalli) (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.
... Smooth Thallus, inly softer you than any furry rabbit, Or glossy goose's oily plumes, or velvet earlap yielding, Or feeble age's heavy ... — The Poems and Fragments of Catullus • Catullus
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