lichen Meaning, Definition & Usage

noun any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks
noun any thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc.

L., fr. Gr. .

(Bot.) One of a class of cellular, flowerless plants, (technically called Lichenes), having no distinction of leaf and stem, usually of scaly, expanded, frond-like forms, but sometimes erect or pendulous and variously branched. They derive their nourishment from the air, and generate by means of spores. The species are very widely distributed, and form irregular spots or patches, usually of a greenish or yellowish color, upon rocks, trees, and various bodies, to which they adhere with great tenacity. They are often improperly called rock moss or tree moss. ✍ A favorite modern theory of lichens (called after its inventor the Schwendener hypothesis), is that they are not autonomous plants, but that they consist of ascigerous fungi, parasitic on algæ. Each lichen is composed of white filaments and green, or greenish, rounded cells, and it is argued that the two are of different nature, the one living at the expense of the other. See Hyphæ, and Gonidia.
(Med.) A name given to several varieties of skin disease, esp. to one characterized by the eruption of small, conical or flat, reddish pimples, which, if unchecked, tend to spread and produce great and even fatal exhaustion.