Lichens occupy a not unimportant place in the economic field. Mites and other small insects, caterpillars and slugs feed on them especially when they are moist and the acids not too pronounced. Petch has stated that they are the staple food of the black termites in Ceylon. Abbe Hue considered that the abundance and perfect development of lichens in the Antarctic was due to the absence of insect life. In northern latitudes sev eral kinds, for example Cladonia alpestris, are of great service as provender for domestic animals. Cladonia rangiferina, the rein deer moss, is the special food of the reindeer. In times of scarcity it has been found advantageous to grind up lichen thalli after elimination of acids, and to mix the powder with meal for human consumption. Lecanora esculenta, a rock lichen and often erratic, is abundant in eastern deserts and has been similarly used : it has been considered that that lichen was the manna of the Israel ites. Species of Umbilicaria and Gyrophora called tripe de roche have been used by Arctic explorers to stay the pangs of hunger. Gyrophora esculenta, an eastern maritime rock lichen is greatly esteemed as an edible plant both in Japan and in China.
Their value in medicine rested in the past on a somewhat fanciful basis—that of the "doctrine of signatures": certain char acteristics of form by their resemblance to organs of the body, were considered to indicate curative properties. Some very bitter species such as Pertusaria faginea served as a substitute for quinine. Cetraria islandica, the "Iceland Moss," owing partly to its gelatinous consistency has been used with good effect in chest troubles, and is now the only lichen recognized in the British Pharmacopeia.
Their use as dye-plants has been known from the earliest times, and before the discovery of aniline dyes the rich and varied col ours obtained from lichens were highly valued. The colouring principle of the dyes is contained in the peculiar lichen-acids. Treatment with an alkali is generally necessary to extract the colour ; mordants are frequently used. With some lichens, boiling
the plants with the material to be dyed is sufficient to secure the desired colour. The dyes can be used only on animal fibres such as wool and silk; they have no effect on linen or cotton. Purple lichen dyes—orchil, litmus or cudbear—are obtained from Roc cella tinctoria, a maritime lichen, Lecanora tartarea and a few others. Other serviceable colours are the varied yellows and browns so much used in home or village industries. But abundant though dye lichens are, they can only furnish a limited quantity and could never meet any large demand.
are discussed in general text-books, espe cially in those devoted to cryptogamic botany. Only the publications concerned exclusively with lichenology are cited here. In most of these will be found lists of books and papers that deal with various aspects of the subject as outlined above.
General: M. Fiinfstiick, Lichenes, A, Allgemciner Teil, in Engler and Prantl's Die Naturlichen Pftanzenfamilien VIII. (Leipzig, 1926) ; Albert Schneider, Text-book of Lichenology (Binghampton, N.Y., 1897) ; A. Lorrain Smith, Lichens (Cambridge Botanical Handbooks, Camb., 1921) ; F. Tobier, Biologie der Flechten (Berlin, 1925) ; W. Zopf, Die Flechten Stolle (Jena, 1907).
Floristic: Incomplete Floras. Among these may be cited Th. Fries, Lichenographia Scandinavica (Uppsala, 1871) ; J. Harmand, Lichens de France, five fascicles (Paris, 1905-13) ; W. Migula, Flora Deutsch lands, Osterreichs and der Schweiz, Abt. II. Krypt. Flora XII. Die Flechten. Lief. I. (in course of publication).