Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/579

 LICHENS 559 Chemical Reactions in Lichens. These have reference to the thallus and the apothecia, and iu both respects afford valuable assist ance in the systematic study of lichens. (a) Thalline reactions depend upon the presence in the thallus of certain colourable materials in the form of acids, and are manifested on the application of hydrate of potash and the hypochlorite of lime either on the eortical stratum or the medulla. Of these reagents, hydrate of potash (K), composed of equal weights of caustic potash and water, and hypochlorite of lime (CaCl), composed of chloride of lime and water of any strength, may give certain reactions or none, according to the presence or absence of particular acids in the thallus. If no reaction takes place, this is denoted by K -, CaCl - , and similarly if there is reaction by K +, CaCl +. A very convenient mode of symbolizing the positive and negative reactions of the cortical Ia3~er and the medulla of the same species is to place the reaction of the former above the latter, e.g., K, CaCl^, denoting that with K the cortical stratum shows reaction and the medulla none, while with CaCl the cortical stratum gives none, but the medulla a dis tinct reaction. Again, there is often no reaction produced by K alone, but if CaCl be added to it while still moist, a decided reaction appears, for which K(CaCl)+ is the symbol; or, on the other hand, the reaction given by K may be neutralized by the immediate application of CaCl, in which case it is expressed by K + (CaCl) -. The positive reactions are due to the presence of par ticular acids in the thallus, such as crythrinic acid giving a crimson reaction with CaCl, clirysoplianic acid giving a purple reaction with K, glaucinic acid giving a yellow reaction with CaCl, and lecanoric acid giving a citrine reaction with K. In most cases where the natural colour of the thallus (often also of the medulla) is yellow or orange, chrysophanic acid is present, and with K gives a purple reaction (e.g., Physcia lychnea), but in others externally similar in colour and general appearance, where only lecithophanic acid is present (e.g., Lecanora laciniosa), K gives no reaction what ever. Sometimes K produces at first a yellow colour which immedi ately changes into a red or purple, as in Lecanora cincrca, which at once enables us to distinguish it from the closely allied species L. gibbosa and L. calcarca. A solution of iodine (I) is also in certain cases useful as a test on the medulla, and in the discrimination of some species of Collema. The value of these chemical tests in lichens was first pointed out by Xylander in Flora, 1866, pp. 223, 224, was subsequently illustrated by him in Flora, 1869, passim, in the case of species belonging to various genera, and has since been generally acknowledged by Hellenists and employed by them in all specific diagnoses. They are unquestionably most important, not only in the discrimination of many difficult and closely allied species, but also in enabling us to refer with certainty varieties to the species to which they properly belong. Nay, even a small frag ment of the thallus, whether sterile or fertile, may by their aid be readily determined, while otherwise it would be either quite indeter minable or doubtful. Generally they may be obtained on any por tion of the thallus, but they are frequently more vivid, as might be expected, in the growing or circumferential portions. It is, however, only immediate reactions which depend on the colouring matter contained in the cortical stratum and the medulla that are to be attended to, and not secondary or tardy reactions which may otherwise originate, e.g., from the dissolution of the chlorophyll of the gonidia. At the same time it is to be observed, in order to prevent a misconception which has occasionally been entertained, that they do not per sc constitute a special specific character, but only an additional and confirmatory specific character. As such their value is clearly apparent in cases where the external characters are similar or approximate, and doubt necessarily exists as to their specific value. (6) Apothecial reactions for the most part take place either externally on the epithecium or internally on the hymenial gelatin. The reactions of the epithecium are generally produced by K on species which have yellow or orange apothecia (e.g., Lecanora aurantiaca, Lecidea Uucoxantha), while others with concolorpus apothecia (e.g., Lecanora epanora, Lecidea lucida] give no reaction whatever, owing to the presence or absence of chryso phanic acid. In certain cases also the reaction with CaCl is very useful in enabling us to separate closely allied species with similar apothecia, e.g., Lecanora subcarnea (epithecium CaCl-) from L. glaucoma (epithecium CaCl + yellow). Again, with respect to the reaction of the hymenial gelatin, this, as already observed, depends on the application of a solution of iodine. The formula by which the solution is to be prepared is iodine, gr. j. ; iodide of potash, gr. iij ; distilled water, ounce. In most lichens, where the spores are mature, this solution will tinge the hymenial gelatin in some cases only blue, in others at first blue and then wine-red, and in others wine-red or tawny wine-coloured only, without any preceding blue tinge. In such instances the reaction obtained is often very useful as a confirmatory specific character. Sometimes, however, the blue reaction takes place only on the thecae (e.g., Pcrtusaria, Cladonia, &c. ), and at other times only on the epispore (e.g., Gra- phis, Thclotrcma, &c. ). In the case of some species belonging to the inferior genera the reaction with I is especially valuable for the assistance it lends in distinguishing them from certain pyrenomy- cetous fungi, to which otherwise they might bo supposed to belong. True, in some fungi (e.g., Peziza) we obtain a reaction with I, and in some lichens we have no reaction visible ; but otherwise in such exceptional instances their respective anatomical characters readily show to which class they belong. Economic Uses of Lichens. These are intimately connected with their chemical con stituents, and are in some respects very important. In the arts, as food and as medicine, many of them have been highly esteemed, though others are not now employed for the same purposes as formerly. 1. Lichens Used in the Arts. Of these the most important are such as yield, by maceration in ammonia, the valuable dyes known in commerce as archil, cudbear, and litmus. These, however, may with propriety be regarded as but different names for the same pigmentary substance, the variations in the character of which are attributable to the different modes in which the pigments are manufactured. Archil proper is derived from several species of Roccella (e.g., R. Montagitei, R. tinctori(i) y which yield a rich purple dye and fetch a high price in the market. Of considerable value is the &quot; perelle &quot; prepared from Lecanora parella, and much used in the preparation of a red or crimson dye. Inferior to this is &quot; cudbear, 3 derived from Lecanora tartarea, which was formerly very extensively employed by the peasantry &quot;of north Europe for giving a scarlet or purple colour to woollen cloths. By adding certain alkalies to the other ingredients used in the preparation of these pigments, the colour becomes indigo- blue, in which case it is the litmus of the Dutch manu facturers. Amongst other lichens affording red, purple, or brown dyes may be mentioned Ramalina scopitlorum, Parmelia saxatilis and P. omphalodes, Vmlilicaria 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 rutpina, 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 colorific 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 prvnostri, and Parmelia pliysodes, 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 mak ing of parchment and cardboard. In the 17th century some filamentose and fruticulose lichens, viz., species of Usnea and Ramalina, also Ever-nia furfur acea and Cladina 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 &quot; Poudre de Cypre &quot; 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 amylaceous substances, and in some cases a small quantity of saccharine