Microscopic Structure—A transverse section exhibits, when strongly magnified, a broad loose central layer of long, thick-walled branching walls of hyphæ, containing air, and enclosing wide hollow spaces. This middle layer encloses a certain number of larger cells called gonidia, coloured with chlorophyll. The gonidia are not destroyed either by strong sulphuric acid, or by boiling them with potash. They assume however a deep violet colour when treated with caustic potash and then left for 24 hours in a solution of iodine in potassium iodide.
The tissues on either side of this central layer consists of very thickly felted hyphæ, without intervening spaces, and does not appear to contain any particular substance. This compact and tenacious tissue passes into a thin cortical layer consisting of cells very closely bound together. Under the influence of reagents this layer becomes very evident: thus when moistened with strong sulphuric or hydrochloric acid, it separates from the rest of the tissue as a coherent membrane, and rolls itself backward. On boiling with water the inner tissue swells up, the cell-walls being partly dissolved. Thin slices of the lichen are coloured reddish or pale blue by iodine water,—more distinctly blue, if previously treated with sulphuric acid. The colour spreads uniformly over the inner tissue, but no starch granules can be detected; the cortical layer is merely coloured brown by iodine. The white spots on the outer surface of the thallus are resolved by pressure under a plate of glass into minute round transparent granules, not coloured by iodine, and thick branched cells like those of the central layer.
The short thick prominences on the edge of the thallus, frequently terminate in one or more sac-like cavities (spermogonia) containing a large number of simple bar-shaped cells (spermatia), only 6 mkm. long; they are enveloped in transparent mucus, and may be expelled by pressure under glass. It has been shown by Stahl (1874) that they represent the fertilizing corpuscles or seaweeds of the class Florideæ.
The observations of De Bary (1866) and Schwendener (1867-70) confirmed and much extended by the researches of Bornet[2745] (1873-74), have shown that the gonidia of lichens are referable to some species of Alga, and are capable of an independent existence; that the relations of the hyphæ to the gonidia are of such a nature as to exclude the possibility of either of those bodies being produced by the other; and further that the theory of parasitism is the only one capable of explaining these relations in a satisfactory manner. Under this singular theory, lichens are compound organisms, formed of an alga, and of a fungus living upon it as a parasite.
Chemical Composition—Boiling water extracts from Iceland moss as much as 70 per cent. of the so-called Lichenin, or Lichen-starch, a body which is perfectly devoid of structure. The decoction (1: 20) gelatinizes on cooling, and assumes a reddish or bluish tint by solution of iodine. This property of lichenin is plainly seen, when the drug is first exhausted by boiling spirit of wine containing some carbonate of potassium; and then boiled with 50 to 100 parts of water, and the decoction precipitated by means of alcohol. The lichenin thus obtained in a purer state, must be deprived of alcohol by cautiously washing it with water. Powdered iodine will now immediately impart to it while still moist an intense blue. Its composition, C₁₂H₂₀O₁₀, agrees with that of starch and cellulose; and it must be regarded as a modification of the latter, being likewise soluble in water and in ammoniacal solution of copper. Lichenin is not a kind of mucilage, because it yields but insignificant traces of mucic acid, if treated with concentrated nitric acid; and also because it contains no inorganic constituents.[2746] The very trifling proportion of mucic acid it furnishes may depend upon the presence, in small amount, of an independent mucilaginous body.
According to Th. Berg (1873), lichenin consists of what he continues to call so, and another constituent, the latter only being coloured by iodine, possessing (dextrogyre) rotatory power, and also being insoluble in ammoniacal solution of copper. Berg’s lichenin is not soluble in cold water, but readily dissolves in hot water, and again separates on cooling. The other constituent on the contrary is abundantly soluble in cold, and very sparingly in hot water. The drug yielded to Berg 20 per cent. of “true” lichenin and 10 per cent. of the other substance.
The chlorophyll of the gonidia is not soluble in hydrochloric acid, and hence is distinguished by Knop and Schnedermann as Thallochlor; its quantity is extremely small.
The bitter principle of Cetraria, called Cetraric Acid or Cetrarin, C₁₈H₁₆O₈, crystallizes in microscopic needles, is nearly insoluble in cold water, and forms with alkalis, yellow, easily soluble, bitter salts. The lichen also contains a little sugar, and about 1 per cent. of a peculiar body, Licheno-stearic Acid, C₁₄H₃₄O₃, the crystals of which melt at 120° C. The Lichenic Acid found by Pfaff in 1826 in Iceland moss, and formerly regarded as a peculiar compound, has been proved identical with fumaric acid.
In common with many lichens, cetraria contains Oxalic Acid and is said to yield also some tartaric acid. The ash, which amounts to 1-2 per cent., consists to the extent of two-fifths of silicic acid combined chiefly with potash and lime.
Collection and Commerce—Iceland moss is collected in many districts where the plant abounds at least for local use, as in Sweden, whence some is shipped to other countries. It is also gathered in Switzerland, especially on the mountains of the Canton of Lucerne, and in Spain.[2747] None is exported from Iceland.