Characters.—Perennial, aerial thallogens, nourished through their whole surface by the medium in which they vegetate; always constituting a thallus, crust, or frond (receptaculum universale; blastema) formed of a cortical and a medullary layer, of which the former is simply cellular, the latter cellular and filamentous. Apothecia (fructus) consisting of a receptacle and a proligerous layer (lamina proligera) composed of spores (sporae) naked or enclosed in spore-cases (asci; thecae), united to form a nucleus or disposed on a disk (discus).
Properties.—The tissue consists of cellulose. Many of them contain amylaceous matters (lichenin or feculoid, and inulin) and their congeners gum and sugar, which render them nutritive, emollient, and demulcent. Bitter principles (cetraric acid; picrolichenin) are sometimes found in lichens; these confer slight tonic properties. Colouring matters (thallochlor, chrysophanic acid, &c.) are frequently present. Colorific principles (orcellic, erythric, lecanoric, and other acids), or principles which, under the combined influence of ammonia and oxygen, form colouring matters (orceine, &c.), render some of the lichens valuable in the arts. Besides the before-mentioned bodies, several other vegetable acids (as the tartaric, oxalic, tannic, and lichestearic) are found in the lichens. The oxalic acid is found in combination with lime: one lichen (Variolaria faginea) contains 47 per cent, of calcareous oxalate. The ashes of the lichens constitute about 8 per cent. of the dried plants, and consist principally of the earths: the ashes of lichens growing on siliceous rocks contain more silica than those of lichens growing in other situations. It is deserving of especial notice that not a single poisonous lichen is known.
The lichens which I shall have to notice may be conveniently divided, according to their uses, into the esculent, the medicinal, and the tinctorial lichens.