A large proportion are esters or alkyl salts formed by the union of an alcohol and an acid; these are insoluble in alkaline carbonates. It is considered probable that the fungus generates the acid, while the alcohol arises in the metabolic processes in the alga. It has indeed been proved that the alcohol, erythrit, is formed in at least two algae, Protococcus vulgaris and Trentepohlia jolithus; and the lichen-acid, erythrin (C₂₀H₂₂O₁₀), obtained from species of Roccella in which the alga is Trentepohlia, is, according to Hesse, the erythrit ester of lecanoric acid (C₁₆H₁₄O₇), a very frequent constituent of lichen thalli. It is certain that the interaction of both symbionts is necessary for acid production. This was strikingly demonstrated by Tobler[820] in his cultural study of the lichen thallus. He succeeded in growing, to a limited extent, the hyphal part of the thallus of Xanthoria parietina on artificial media; but the filaments remained persistently colourless until he added green algal cells to the culture. Almost immediately thereafter the characteristic yellow colour appeared, proving the presence of parietin, formerly known as chrysophanic acid. Tobler’s observation may easily be verified in plants from natural habitats. A depauperate form of Placodium citrinum consisting mainly of a hypothallus of felted hyphae, with minute scattered granules containing algae, was tested with potash, and only the hyphae immediately covering the algal granules took the stain; the hypothallus gave no reaction.
It has been suggested[821] that when a decrease of albumenoids takes place, the quantity of lichen-acid increases, so that the excreted substance should be regarded as a sort of waste product of the living plant, “rather than as a product of deassimilation.” The subject is not yet wholly understood.
d. Causes of Variation in Quantity and Quality of Lichen-Acids. Though it has been proved that lichen-acids are formed freely all the year round on any soil or in any region, it happens occasionally that they are almost or entirely lacking in growing plants. Schwarz[822] found this to be the case in certain plants of Lecanora tartarea, and he suggests that the gyrophoric acid contained in the outer cortex of that lichen had been broken up by the ammonia of the atmosphere into carbonic acid and orcin which is soluble in water, and would thus be washed away by rain. It has also been shown by Schwendener[823] and others that the outer layers of the older thallus in many lichens slowly perish, first breaking up and then peeling off; the denuded areas would therefore have lost, for some time at least, their particular acids. Fünfstück[824] considers that the difference in the presence and amount of acid in the same species of lichen may be due very often to variation in the chemical character of the substratum, and this view tallies with the results noted by Heber Howe[825] in his study of American Ramalinae. He observed that, though all showed a pale-yellow reaction with potash, those growing on mineral substrata gave a more pronouncedly yellow colour.
M. C. Knowles[826] found that in Ramalina scopulorum the colour reaction to potash varied extremely, being more rapid and more intense, the more the plants were subject to the influence of the sea-spray.
Lichen-acids are peculiarly abundant in soredia, and as, in some species, the thallus forms these outgrowths, or even becomes leprose more freely in damp weather, the amount of acids produced may depend on the amount of moisture in the atmosphere.
Their formation is also strongly influenced by light, as is well shown by the varying intensity of colour in some yellow thalli. Placodium elegans, always a brightly coloured lichen, changes from yellow to sealing-wax red in situations exposed to the full blaze of the sun. Haematomma ventosum,though greenish-yellow in lowland situations is intensely yellow in the high Alps. The same variation of colour is characteristic of Rhizocarpon geographicum which is a bright citron-yellow at high altitudes, and becomes more greenish in hue as it nears the plains. The familiar foliose lichen Xanthoria parietina is a brilliant orange-yellow in sunny situations, but grey-green in the shade, and then yielding only minute quantities of parietin. West[827] and others have noted its more luxuriant growth and brighter colour when it grows in positions where nitrogenous food is plentiful, such as the roofs of farm-buildings, which are supplied with manure-laden dust, and boulders by the sea-shore frequented by birds.
e. Distribution of Acids. Some acids, so far as is known, are only to be found in one or at most in very few lichens, as for instance cuspidatic acid which is present in Ramalina cuspidata, and scopuloric acid, a constituent of Ramalina scopulorum, the acids having been held to distinguish by their reactions the one plant from the other.
Others of these peculiar products are abundant and widely distributed. Usninic acid, one of the commonest, has been determined in some 70 species belonging to widely diverse genera, and atranorin, a substance first discovered in Lecanora atra, has been found again many times; Zopf gives a list of about 73 species or varieties from which it has been extracted. Another widely distributed acid is salazinic acid which has been found by Lettau[828] in a very large number of lichens.
E. Chemical Grouping of Lichen-Acids
Most of these acids have been provisionally arranged by Zopf in groups under the two great organic series: I. The Fat series; and II. The Benzole or Aromatic series.