A. Lichens as Food.
a. Food for Insects, etc. Some of the earlier botanists made careful observations on the important place occupied by lichens in nature as affording food to many small animals. In 1791 Jacques Brez[1226] wrote his Flore des Insectophyles, and in the list of food-plants he includes seven species of lichens. The “insects” that frequented these lichens were species of the genera Acarus (mites) and Phalena (moths). A few years later Persoon[1227] noted that lichens formed the main food supply of many insects, slugs, etc. Zukal[1228], quoting from Otto Wilde (Die Pflanzen und Raupen Deutschlands, Berlin, 1860), gives a list of caterpillars that are known to feed on and destroy lichens.
A very considerable number of small creatures feed eagerly on lichens, and traces of their depredations are constantly to be seen in the empty fruit discs, and in the cortices eaten away in patches so as to expose the white medulla. It has been argued by Zukal[1229] that the great formation of acid substances in lichens is for shielding them against the attacks of animals; Zopf[1230] on the contrary insists that these substances afford the plants no real protection. He made a series of experiments with snails, feeding them with slices of potato smeared with pure lichen acids. Many snails ate the slices with great readiness even when covered with bitter acids such as cetraric, or with those which are poisonous for other animals such as rhizocarpic and pinastrinic. The only acid they refused was vulpinic, which is said to be poisonous for vertebrates. The crystals of the acids passed unchanged through the alimentary canal of the snails, and were found in masses in the excreta. They were undissolved, but, enclosed in slime, their sharp edges did no damage to the digestive tract.
Stahl[1231] however upholds Zukal’s theory of the protective function of lichen acids against the attacks of small animals. Some few snails, caterpillars, etc., that are omnivorous feeders consume most lichens with impunity, and the bitter taste seems to attract rather than repel them; but many others he contends are certainly prevented from eating lichens by the presence of the acids. He proved this by soaking portions of the thalli of certain bitter species for about twenty-four hours in a one per cent. soda solution, which was sufficiently strong to extract the acids. He found that these treated specimens were in most cases preferred to fresh portions that had been simply moistened with water.
Even the omnivorous snail, Helix hortensis, was several times observed to touch the fresh thallus and then creep away, while it ate continuously the soda-washed portion as soon as it came into contact with it. Calcium Oxalate, on the other hand, formed no protection; omnivorous feeders ate indifferently calcicolous lichens such as Aspicilia calcarea and Lecanora saxicola, whether treated with soda or not, but would only accept lichens with acid contents, such as Parmelia caperata, Evernia prunastri, etc., after they had been duly soaked.
Experiments were also made with wood-lice (Oniscus murarius), and with earwigs (Forficula auricularia), and the result was the same: they would only eat bitter lichens after the acids had been extracted by the soda method. Stahl therefore concludes that acids must be regarded as eminently adapted to protect lichens which otherwise, owing to their slowness of growth, would scarcely escape extinction.
The gelatinous Collemaceae, as also Nostoc, the alga with which these are associated, are unharmed by snails, etc., on account of their slippery consistency when moist, which prevents the creatures from getting a foothold on the thallus. These lichens however do not contain acids, and if, when dry, they are reduced to powder and then moistened, they are eagerly eaten both by snails and by wood-lice. Peltigera canina, on account of a disagreeable odour it acquires on being chewed, is avoided to a certain extent, but even so it is frequently found with much of the thallus eaten away.
Hue[1232] in his study of Antarctic lichens, comments on the abundance and perfect development of the lichens, especially the crustaceous species, which cover every inch of rock surface. He ascribes this to the absence of snails and insects which in other regions so seriously interfere with the normal and continuous growth of these plants.
Snails do not eat lichens when they are dry and hard, but on damp or dewy nights, and on rainy days, all kinds, both large and small, come out of their shells and devour the lichen thalli softened by moisture. Large slugs (Limax) have been seen devouring with great satisfaction Pertusaria faginea, a bitter crustaceous lichen. The same Limax species eats many different lichens, some of them containing very bitter substances. Zopf[1233] observed that Helix cingulata ate ten different lichens, containing as many different kinds of acid.
Other creatures such as mites, wood-lice, and the caterpillars of many butterflies live on lichens, though, with the exception of the caterpillars, they eat them only when moist. Very frequently the apothecial discs and the soredia are taken first as being evidently the choicest portions. All lichens are, however, not equally palatable. Bitter[1234] observed that the insect Psocus (Orthoptera) had a distinct preference for certain species, and restricted its attention to them probably because of their chemical constitution. He noted that in a large spreading thallus of Graphis elegans on holly, irregular bare spots appeared, due to the ravages of insects—probably Psocus. In other places, the thallus alone had been consumed, leaving the rather hard black fruits (lirellae) untouched. In time the thallus of Thelotrema lepadinum, also a crustaceous lichen, invaded the naked areas, and surrounded the Graphis lirellae. The new comer was not to the taste of the insects and was left untouched.
Petch[1235] says that lichens form the staple food of Termes monoceros, the black termite of Ceylon. These ants really prefer algae, but as the supply is limited they fall back on lichens, though they only consume those of a particular type, or at a particular stage of development. Those with a tough smooth cortex are avoided, preference being given to thalli with a loose powdery surface. At the feeding ground the ants congregate on the suitable lichens. With their mandibles they scrape off small fragments of the thallus which they form into balls, varying in size from 1·5 mm. to 2·5 mm. in diameter. The workers then convey these to the nests in their mandibles. It would seem that they carry about these balls of food, and allow the ants busy in the nest to nibble off portions. Lichen balls are not used by termites as fungi are, for “gardens.”
Other observations have been made by Paulson and Thompson[1236] in their study of Epping Forest lichens: “Mites of the family Oribatidae must be reckoned among the chief foes of these plants upon which they feed, seeming to have a special predilection for the ripe fruits. We have had excellent specimens of Physcia parietina spoiled by hidden mites of this family, which have eaten out the contents of the mature apothecia after the lichens have been gathered. One can sometimes see small flocks of the mites browsing upon the thallus of tree-dwelling lichens, like cattle in a meadow.” The Oribatidae, sometimes called beetle-mites, a family of Acarinae, are minute creatures familiar to microscopists. They live chiefly on or about mosses, but Michael[1237] is of opinion that a large number frequent these plants for the fungi and lichens which grow in and about the mosses. In Michael’s Monograph of British Oribatidae, four species are mentioned as true lichen-lovers, Leiosoma palmicinetum found on Peltigera canina and allied species; Cepteus ocellatus and Oribata parmeliae which live on Physciae, the latter exclusively on Physcia (Xanthoria) parietina; and Scutovertes maculatus which confines itself to lichens by the sea-shore. Another species, Notaspis lucorum, frequents maritime lichens, but it is also found on other substrata; while Tegeocranus labyrinthicus, though usually a lichen-eating species, lives either on mosses or on lichens on walls. Zopf[1238] reckoned twenty-nine species of lichens, mostly the larger foliose and fruticose kinds, that were eaten by mites. Lesdain[1239] in his observations on mite action notes that frequently the thallus round the base of the perithecia of Verrucaria sp. was eaten clean away, leaving the perithecia solitary and extremely difficult to determine.
Fig. 126. 1, Tetranychus lapidus, enlarged; 2, Verrucaria calciseda with eggs in situ, slightly enlarged; 3 and 4, eggs attached to lichen fruits, much magnified (after Wheldon).
J. A. Wheldon[1240] found the eggs of a species of mite, Tetranychus lapidus, attached to the fruits of Verrucaria calciseda, Lecidea immersa and L. Metzleri, calcicolous lichens of which the thallus not only burrows deep down into the limestone, but the fruits form in shallow excavated pits ([Fig. 126]). The eggs of this stone mite are found fairly frequently on exposed limestone rocks, bare of vegetation, except for a few crustaceous lichens. “There is usually a single egg, rarely two, in each pit apparently attached to the old lichen apothecium. The eggs are very attractive objects under a lens; they measure ·5 mm. in diameter, and are disc-like with a central circular depression from which numerous ridges radiate to the circumference, like the spokes of a wheel. When fresh, they have a white pearly lustre, becoming chalk-white when dry and old.” Wheldon’s observations were made in the Carnforth and Silverdale district of West Lancashire.
A minute organism, Hymenobolina parasitica[1241], first described by Zukal and doubtfully grouped among the mycetozoa, feeds, in the plasmodium stage, on living lichens. The parasitic habit is unlike that of true mycetozoa. It has recently been recorded from Aberdeenshire.
b. Insect mimicry of Lichens. Paulson and Thompson[1242] give instances of moth caterpillars, which not only feed on lichens, but which take on the coloration of the lichens they affect, either in the larval or in the perfect moth stage. “One of the most remarkable examples of this protective resemblance to lichens is that of the larva of the geometrid moth, Cleora lichenaria, which feeds upon foliose lichens growing upon tree-trunks and palings, and being of a green-grey hue, and possessed of two little humps on many of their body-segments, they so exactly resemble the lichens in colour and appearance as to be extremely difficult of detection.” Several instances are recorded of moths that resemble the lichens on which they settle: perfect examples of such similarity are exhibited at the Natural History Museum, South Kensington, where Teras literana, Moma orion, and other moths are shown at rest on lichen-covered bark from which they can hardly be distinguished.
Another curious instance of suggested mimicry is recorded by G.E. Stone[1243]. He spotted a number of bodies on the bark of some sickly elms in Massachusetts. They were about 1/8 of an inch in diameter “with a dark centre and a drab foliaceous margin.” They were principally lodged in the crevices of the bark and Stone collected them under the impression that they were the apothecia of a lichen most nearly resembling those of Physcia hypoleuca. Some of the bodies were even attached to the thallus of a species of Physcia; others were on the naked bark and had every appearance of lichen fruits. Only closer examination proved their insect nature, and they were identified as belonging to a species Gossypina Ulmi, an elm-leaf beetle common in Europe where it causes a disease of the tree. It had been imported into the United States and had attacked American elms.
It is stated by Tutt[1244] that the larvae of many of the Psychides (Lepidoptera) live on the lichens of trees and walls, such as Candelaria concolor, Xanthoria parietina, Physcia pulverulenta and Buellia canescens, and that their larvae pupate on their feeding grounds. Each species makes a “case” peculiar to itself, but those of the lower families are usually covered externally with grains of sand, scraps of lichens, etc. The “case” of Narcyria monilifera, for instance, is somewhat raised on a flat base and is obscured with particles of sand and yellow lichen, giving the whole a yellow appearance. That of Luffia lapidella is roughly conical and is held up at an angle of 30° to 45° when the larva moves. The “cases” of Bacotia sepium are always upright; they measure about 5·5 mm. in height and 2·75 mm. in width and present a hoary appearance from the minute particles of lichen with which they are covered, so that the structure is not unlike the podetium of a Cladonia.
c. Food for the Higher Animals. It has been affirmed, especially by Henneguy, that many lichens, if deprived of the bitter principle they contain, by soaking in water, or with the addition of sodium or potassium carbonate, might be used with advantage as fodder for animals. He cites as examples of such, Lobaria pulmonaria, Evernia prunastri, Ramalina fraxinea, R. farinacea, and R. fastigiata, all of which grow abundantly on trees, and owe their nutritive quality to the presence of lichenin, a carbohydrate allied to starch.
Fig. 127. Cladonia rangiferina Web. (S. H., Photo.).
Cladonia rangiferina ([Fig. 127]), the well-known “reindeer moss,” is, however, the lichen of most economic importance, as food for reindeer, cattle, etc. It is a social plant and forms dense tufts and swards of slender, much branched, hollow stalks of a greenish-grey colour which may reach a height of twelve inches or even more; the stalks decay slowly at the base as they increase at the apex, so that very great length is never attained. In normal conditions they neither wither nor die, and growth continues indefinitely. It is comparatively rare in the northern or hilly regions of the British Isles, and is frequently confused with the somewhat smaller species Cl. sylvatica which is very common on our moorlands, a species which Zopf[1245] tells us reindeer absolutely refuse to eat.
The true reindeer moss is abundant in northern countries, more especially in forest regions[1246] and in valleys between the tundra hills which are more or less sheltered from the high winds; it is independent of the substratum and flourishes equally on barren sand and on wet turf; but grows especially well on soil devastated by fire. For long periods it may be covered with snow without injury and the reindeer are accustomed to dig down with horns and hoofs in order to reach their favourite food. Though always considered as peculiarly “reindeer” moss, deer, roebuck and other wild animals, such as Lemming rats[1247], feed on it largely during the winter. In some northern districts it is collected and stored as fodder for domestic cattle; hot water is poured over it and it is then mixed with straw and sprinkled with a little salt. Johnson[1248] has reported that the richness of the milk yielded by the small cows of Northern Scandinavia is attributed by some to their feeding in great measure on the “reindeer moss.”
Fig. 128. Cetraria islandica Ach. (S. H., Photo.).
When Cladonia rangiferina is scarce, a few other lichens[1249] are made use of, Alectoria jubata, a brownish-black filamentous tree-lichen being one of the most frequent substitutes. Stereocaulon paschale, which grows in large dense tufts on the ground in mountainous regions, is also eaten by reindeer and other animals; and Iceland moss, Cetraria islandica, is stored up in large quantities by the Icelanders and used as fodder. Willemet[1250] reports it as good for horses, oxen, cows and pigs.
It is interesting to recall a discovery of prehistoric remains at the Abbey of Schussenried on the Lake of Constance and described by F. Keller[1251]: under successive beds of peat and crumbly tufa, there was found a layer, 3 feet thick, containing flints, horns of reindeer and bones of various animals, and, along with these, masses of reindeer moss; a sufficient proof of its antiquity as a fodder-plant.
d. Food for Man. Lichens contain no true starch nor cellulose, but the lichenin present in the cell-walls of the hyphae has long been utilized as a food substance. It is peculiarly abundant in Cetraria islandica ([Fig. 128]), which grows in northern countries, covering great stretches of ground with its upright strap-shaped branching fronds of varying shades of brown. In more southern lands it is to be found on high hills or on upland moors, but in much smaller quantities. Commercial “Iceland moss” is supplied from Sweden, Norway or Iceland. In the last-named country the inhabitants harvest the lichen preferably from bare stony soil where there is no admixture of other vegetation. They revisit the locality at intervals of three years, the time required for the lichen to grow to a profitable size; and they select the wet season for the ingathering of the plants as they are more easily detached when they are wet. If the weather should be dry, they collect it during the night. When gathered it is cleansed from foreign matter and washed in water to remove as much as possible of the bitter principle. It is then dried and reduced to powder. When required, the powder is put to macerate in water for 24 hours, or it is soaked in a weak solution of soda or of carbonate of potassium, by which means the bitter cetraric acid is nearly all eliminated. When boiled[1252] it yields a jelly which forms the basis of various light and easily digested soups or of other delicacies prepared by boiling in milk, which have been proved to be valuable for dyspeptics or sufferers from chest diseases. The northern nations also make the powder into bread, porridge or gruel. Johnson[1253] states in his account of “Useful Plants” that considerable quantities of Iceland moss were formerly employed in the manufacture of sea biscuit, and that ship’s bread mixed with it was said to be less liable to the attacks of weevil than when made from wheat flour only.
An examination of the real food value of the mucilaginous extract from “Iceland moss” has been made by several workers. Church[1254] states that for one part of flesh formers, there are eight parts of heat-givers reckoned as starch. Brown[1255] isolated the two carbohydrates, lichenin and isolichenin. The former, a jelly which yields on hydrolysis a large quantity of a reducing sugar, dextrose, ferments with yeast and gives no phloroglucin reaction; it is unaffected by digestion and probably does not form glycogen. Iso-lichenin is much less abundant and resembles soluble starch, but on digestion yields only dextrins—no sugar. It may be concluded, judging from the chemical nature of the mucilage, from the resistance of its constituents to digestion and from the small amount present in the jelly, that its nutritive value is practically nil[1256].
It has been stated that “reindeer moss” in times of food scarcity is powdered and mixed with “Iceland moss” and rye to make bread in North Finland. Johnson confirms this and cites the evidence of a Dr Clarke that: “to our surprise we found we might eat of it with as much ease as of the heart of a fine lettuce. It tasted like wheat-bran, but after swallowing it, there remained in the throat and upon the palate a gentle heat, or sense of burning, as if a small quantity of pepper had been mixed with the lichen.”
The Egyptians[1257] have used Evernia prunastri, more rarely E. furfuracea, in baking. In the eighteenth century fermentative agents such as yeast were unknown to them, and these lichens, which were imported from more northern lands, were soaked in water for two hours and the solution then mixed with the flour to give a much appreciated flavour to the unleavened bread.
In India[1258] a species of Parmelia (near to P. perlata) known in the Telegu language as “rathapu” or rock-flower has been used as a food, generally prepared as a curry, by the natives in the Bellary district (Madras Presidency), and is esteemed as a delicacy. It is also used medicinally. The collecting of rathapu is carried on during the hot weather in April and May, and forms a profitable business.
A note has been published by Calkins[1259], on the authority of a correspondent in Japan, that large quantities of Endocarpon (Dermatocarpon) miniatum ([Fig. 56]) are collected in the mountains of that country for culinary purposes, and largely exported to China as an article of luxury. The local name is “iwataka,” meaning stone-mushroom. Properly prepared it resembles tripe. It is possibly the same lichen under a different name, Gyrophora esculenta, which is described by Manabu Miyoshi[1260] as of great food value in Japan where it is known as “iwatake.” It is a greyish-brown leathery “monophyllous” plant of somewhat circular outline and fairly large size, measuring 3 to 13 cm. across. Fertile specimens are rare, and are smaller than the sterile. It grows generally on the steep declivities of damp granitic rocks and is common in various districts of Japan, being especially abundant on such mountains as Kiso, Nikko, Kimano, etc. The face of the precipices is often thickly covered with the lichen growth. The inhabitants collect the plants in large quantities. They dry them and send them to the towns, where they are sold in all vegetable stores; some are even exported to other countries. These lichens are not bitter to the taste, nor are they irritating as are other species of the genus. They are on the contrary quite harmless and are much relished by the Japanese on account of their agreeable flavour, in spite of their being somewhat indigestible. Though only determined scientifically in recent times, this edible lichen has long been known, and the risks attending its collection have frequently been described in Old Chinese and Japanese writings.
Fig. 129. Gyrophora polyrhiza Koerb. (S. H., Photo., reduced).
Other species of Gyrophora including G. polyrhiza ([Fig. 129]) and Umbilicaria, black leathery lichens which grow on rocks in northern regions, have also been used as food. They are the “Tripe de Roche” or Rock Tripe of Arctic regions, a name given to the plants by Canadian fur-hunters. They have been eaten by travellers and others in desperate straits for food; but though to a certain extent nutritious, they are bitter and nauseous, and cause severe internal irritation if the bitter acids are not first extracted by boiling or soaking.
Of more historical interest is the desert lichen Lecanora esculenta, supposed to be the manna[1261] of the Israelites, and still called “bread from heaven.” Eversmann[1262] wrote an account of its occurrence and qualities, and fuller information was given by Berkeley[1263]: when mixed with meal to a third of its weight it is made into bread and eaten by the desert tribes. It grows abundantly in North Africa and in many parts of Western Asia, on the rocks or on soil. It is easily broken off and driven into heaps by the wind; and has been reported as covering the soil to a depth of 15 cm. to 20 cm. with irregular contorted lumps varying in size from a pea to a small nut ([Fig. 130]). Externally these are clear brown or whitish; the interior is white, and consists of branching interlaced hyphae, with masses of calcium oxalate crystals, averaging about 60 per cent. or more of the whole substance.
Fig. 130. Lecanora esculenta Eversm. Loose nodules of the sterile thallus.
A still more exhaustive account is given by Visiani[1264], who quotes the experience of a certain General Jussuf, who had tested its value in the Sahara as food for his soldiers. When bread was made from the lichen alone it was friable and without consistency; when mixed with a tenth portion of meal it was similar to the soldiers’ ordinary bread, and had something of the same taste. The General also gave it as fodder to the horses, some of them being nourished with the lichen and a mixture of barley for three weeks without showing any ill effects. It is also said that camels, gazelles and other quadrupeds eat it with advantage, though it is in any case a very defective food.
A remarkable deposit of the lichen occurred in recent times in Mesopotamia during a violent storm of hail. After the hail had melted, the ground was seen to be covered, and specimens were sent to Errera[1265] for examination. He identified it as Lecanora esculenta. In his opinion two kinds of manna are alluded to in the Bible: in one case (Exodus xvi.) it is the sweet gum exuded from the tamarisk that is described; the other kind (Numbers xi.), he thinks, plainly refers to the lichen. He considers that its nutritive value must be very low, and it can only be valued as food in times of famine.