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Rh in the preparation of these pigments, the colour becomes indigo-blue, in which case it is the litmus of the Dutch manufacturers. Amongst other lichens affording red, purple or brown dyes may be mentioned Ramalina scopulorum, Parmelia, saxatilis and P. amphalodes, Umbilicaria pustulata and several species of Gyrophora, Urceolaria scruposa, all of which are more or less employed as domestic dyes. Yellow dyes, again, are derived from Chlorea vulpina, Platysma juniperinum, Parmelia caperata and P. conspersa, Physcia flavicans, ''Ph. parietina and Ph.'' lychnea, though like the preceding they do not form articles of commerce, being merely used locally by the natives of the regions in which they occur most plentifully. In addition to these, many exotic lichens, belonging especially to Parmelia and Sticta (e.g. Parmelia tinctorum, Sticta argyracea), are rich in colouring matter, and, if obtained in sufficient quantity, would yield a dye in every way equal to archil. These pigments primarily depend upon special acids contained in the thalli of lichens, and their presence may readily be detected by means of the reagents already noticed. In the process of manufacture, however, they undergo various changes, of which the chemistry is still but little understood. At one time also some species were used in the arts for supplying a gum as a substitute for gum-arabic. These were chiefly Ramalina fraxinea, Evernia prunastri and Parmelia physodes, all of which contain a considerable proportion of gummy matter (of a much inferior quality, however, to gum-arabic), and were employed in the process of calico-printing and in the making of parchment and cardboard. In the 17th century some filamentose and fruticulose lichens, viz. species of Usnea and Ramalina, also Evernia furfuracea and Cladonia rangiferina, were used in the art of perfumery. From their supposed aptitude to imbibe and retain odours, their powder was the basis of various perfumes, such as the celebrated “Poudre de Cypre” of the hairdressers, but their employment in this respect has long since been abandoned.

2. Nutritive Lichens.—Of still greater importance is the capacity of many species for supplying food for man and beast. This results from their containing starchy substances, and in some cases a small quantity of saccharine matter of the nature of mannite. One of the most useful nutritious species is Cetraria islandica, “Iceland moss,” which, after being deprived of its bitterness by boiling in water, is reduced to a powder and made into cakes, or is boiled and eaten with milk by the poor Icelander, whose sole food it often constitutes. Similarly Cladonia rangiferina and Cl. sylvatica, the familiar “reindeer moss,” are frequently eaten by man in times of scarcity, after being powdered and mixed with flour. Their chief importance, however, is that in Lapland and other northern countries they supply the winter food of the reindeer and other animals, who scrape away the snow and eagerly feed upon them. Another nutritious lichen is the “Tripe de Roche” of the arctic regions, consisting of several species of the Gyrophorei, which when boiled is often eaten by the Canadian hunters and Red Indians when pressed by hunger. But the most singular esculent lichen of all is the “manna lichen,” which in times of drought and famine has served as food for large numbers of men and cattle in the arid steppes of various countries stretching from Algiers to Tartary. This is derived chiefly from Lecanora esculenta, which grows unattached on the ground in layers from 3 to 6 in. thick over large tracts of country in the form of small irregular lumps of a greyish or white colour. In connexion with their use as food we may observe that of recent years in Scandinavia and Russia an alcoholic spirit has been distilled from Cladonia rangiferina and extensively consumed, especially in seasons when potatoes were scarce and dear. Formerly also Sticta pulmonaria was much employed in brewing instead of hops, and it is said that a Siberian monastery was much celebrated for its beer which was flavoured with the bitter principle of this species.

3. Medicinal Lichens.—During the middle ages, and even in some quarters to a much later period, lichens were extensively used in medicine in various European countries. Many species had a great repute as demulcents, febrifuges, astringents, tonics, purgatives and anthelmintics. The chief of those employed for one or other, and in some cases for several, of these purposes were Cladonia pyxidata, Usnea barbata, Ramalina farinacea, Evernia prunastri, Cetraria ìslandica, Sticla pulmonaria, Parmelia saxatilis, Xanthoria parietina and Pertusaria amara. Others again were believed to be endowed with specific virtues, e.g. Peltigera canina, which formed the basis of the celebrated “pulvis antilyssus” of Dr Mead, long regarded as a sovereign cure for hydrophobia; Platysma juniperinum, lauded as a specific in jaundice, no doubt on the similia similibus principle from a resemblance between its yellow colour and that of the jaundiced skin; Peltidea aphthosa, which on the same principle was regarded by the Swedes, when boiled in milk, as an effectual remedy for the aphthae or rash on their children. Almost all of these virtues, general or specific, were imaginary; and at the present day, except perhaps in some remoter districts of northern Europe, only one of them is employed as a remedial agent. This is the “Iceland moss” of the druggists’ shops, which is undoubtedly an excellent demulcent in various dyspeptic and chest complaints. No lichen is known to be possessed of any poisonous properties to man, although Chlorea vulpina is believed by the Swedes to be so. Zukal has considered that the lichen acids protect the lichen from the attacks of animals; the experiments of Zopf, however, have cast doubt on this; certainly lichens containing very bitter acids are eaten by mites though some of the acids appear to be poisonous to frogs.

The dual nature of the lichen thallus introduces at the outset a classificatory difficulty. Theoretically the lichens may be classified on the basis of their algal constituent, on the basis of their fungal constituent, or they may be classified as if they were homogeneous organisms. The first of these systems is impracticable owing to the absence of algal reproductive organs and the similarity of the algal cells (gonidia) in a large number of different forms. The second system is the most obvious one, since the fungus is the dominant partner and produces reproductive organs. The third system was that of Nylander and his followers, who did not accept the Schwenderian doctrine of duality. In actual practice the difference between the second and third methods is not very great since the fungus is the producer of the reproductive organs and generally the main constituent. Most systems agree in deriving the major divisions from the characters of the reproductive organs (perithecia, apothecia, or basidiospore bearing fructification), while the characters of the algal cells and those of the thallus generally are used for the minor divisions. The difference between the various systems lies in the relative importance given to the reproductive characters on the one hand and the vegetative characters on the other. In the system (1854-1855) of Nylander the greater weight is given to the latter, while in more modern systems the former characters receive the more attention.

A brief outline of a system of classification, mainly that of Zahlbruckner as given in Engler and Prantl’s Pflanzenfamilien, is outlined below.

There are two main divisions of lichens, Ascolichenes and Basidiolichenes, according to the nature of the fungal element, whether an ascomycete or basidiomycete. The Ascolichenes are again divided into Pyrenocarpeae or Pyrenolichenes and Gymnocarpeae or Discolichenes; the first having an ascocarp of the nature of a perithecium, the second bearing their ascospores in an open apothecium.

Series I. Perithecium simple not divided.
 * a. With Pleurococcus or Palmella gonidia. Moriolaceae, Verrucariaceae, Pyrenothamnaceae.
 * b. With Chroolepus gonidia. Pyrenulaceae, Paratheliaceae.
 * c. With Phyllactidium or Cephaleurus gonidia. Strigulaceae.
 * d. With Nostoc or Scytonema gonidia. Pyrenidiaceae.

Series II. Perithecia divided or imperfectly divided by cross-walls.
 * Mycoporaceae with Palmella or Chroolepus gonidia.