This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Iceland moss, Cetraria islandica, Acharius (Class, Fungi; Subclass, Ascomycetes; Order, Discomycetes), is a foliaceous lichen indigenous to Great Britain and widely distributed over the northern hemisphere. It is collected chiefly in Sweden and Central Europe, growing usually amidst moss and grass on the lower mountain slopes.
The lichen consists of a very thin, erect, leafy thallus, branching fanlike into curled or flattened papery lobes about 6 mm. broad, fringed with minute projections each of which terminates in a spermagonium containing numerous spermatia capable of reproducing the plant. It is remarkably harsh and springy to the touch, tough when slightly moist, but brittle when quite dry. The upper surface is usually of a brownish or greenish brown colour; the under surface greyish and marked with numerous small, white, depressed spots. The apothecia are circular, of a dark reddish brown colour, and about 5 mm. in diameter; they are not often to be found on the plant. The drug is almost odourless, and has, when chewed, a mucilaginous, bitter taste. A decoction (1 to 20) yields on cooling a jelly which is stained blue by iodine
The principal constituent of Iceland moss is the carbohydrate lichenin, C6H10O5, which is accompanied by isolichenin (dextrolichenin, Fluckiger). Isolichenin is soluble in cold water, and behaves as a soluble modification of starch. Lichenin dissolves in boiling water, but the solution gelatinises on cooling. Both lichenin and isolichenin are converted by hydrolysis with dilute mineral acids into dextrose; as much as 72 per cent. of fermentable sugar can by this means be obtained from the drug.
Fig. 114. - Iceland Moss (Cetraria islandica.) Natural size. (Luerssen).
Iceland moss also contains cetraric acid, C20H18O9, a crystalline bitter substance, almost insoluble in water, but forming soluble acid salts with monovalent alkalies; hence the bitterness of the drug can . be removed by soaking in dilute solution of sodium bicarbonate. Other constituents of the drug are bitter protocetraric acid and tasteless lichenostearic acid.
The properties of Iceland moss are those of a bitter tonic and nutritive, but it is now seldom employed.
Litmus is a colouring matter obtained from various lichens, chiefly Roccella tinctoria, de Candolle (Cape Verde). R. Montagnei, Bel. (Madagascar), Dendrographa leucophoea, Darbish, etc. (N.O. Discomycetes). The method of preparation is guarded as a trade secret, but it appears to depend mainly upon the slow fermentation of the soaked and ground lichen in the presence of ammonium and potassium carbonates. A red colour is first produced which gradually changes to blue. The blue liquid is drawn off and evaporated, with the addition of chalk and gypsum; the mass is then cut into small rectangular cakes and dried. The chief constituents are erythrolitmin and azolitmin, together with erythrolein and spaniolit-min. The lichens themselves contain lecanoric acid, erythrin and orcin. By the action of alkalies, these yield orsellinic acid. Orsellinic acid by further change yields orsin, from which, by oxidation in the presence of ammonia the colouring matters are produced.
Cudbear (Persio) is a reddish colouring matter prepared by an analogous method from the same lichens.