Lichens

In Ireland, a thorough examination has been made of a rocky coast at Howth near Dublin by M. C. Knowles[1187]. She recognizes five distinct belts beginning with those furthest from the shore though within the influence of the salt water:

• 1. The Ramalina belt.
• 2. The Orange belt.
• 3. Lichina Vegetation.
• 4. Verrucaria maura belt.
• 5. The belt of Marine Verrucarias.

(1) The Ramalina belt. In this belt there are two zones of lichen vegetation: those in the upper zone consist mainly of barren plants of Ramalina siliquosa[1188], rather dark or glaucous in colour with much branched fronds which are incurved at the tips (Fig. 122). They are beyond the direct action of the waves. The lower zone consists also mainly of the same Ramalina, the plants bearing straight, stiff, simple, or slightly branched fertile fronds of a pale-green or straw colour (Fig. 123). The pale colour may be partly due to frequent splashings by sea-spray.

Ramalina siliquosum in both zones takes several distinct forms, according to exposure to light, wind or spray, the effects of which are most marked in the upper zone. The plants growing above the ordinary spray zone generally form sward-like growths (Fig. 124); at the higher levels the sward growth is replaced by isolated tufts with a smaller more amorphous thallus which passes into a very small stunted condition. The latter form alone has gained and retained a footing on the steep faces of the hard and close-grained quartzite rocks. “On the western faces, indeed, it is the only visible vegetation.” The dwarfed tufts with lacerated fronds measuring from 1/4 to 1/2 an inch in height are dotted all over the quartzites. On the sea faces the plants are larger, but everywhere they are closely appressed to the rock surface. At lower levels the fronds lengthen to more normal dimensions. “On these steep rock-faces there is a complete absence of any of the crustaceous species. The problem, therefore, as to how the Ramalina has obtained a foothold on these very hard precipitous rocks, which are too inhospitable even for crustaceous species is an interesting and puzzling one.”

In the Ramalina zone along with the dominant species there occur occasional tufts of R. Curnowii and R. subfarinacea, the latter more especially in shady and rather moist situations. There are also numerous foliaceous and crustaceous lichens mingling with the Ramalina vegetation (Fig. 125), several Parmelias, Physcia aquila, Xanthoria parietina, Buellia canescens, B. ryssolea, Lecanora atra, L. sordida, Rhizocarpon geographicum and others. In the main these are arranged in the following order descending towards the sea:

• 1. Parmeliae.
• 2. Physcia aquila.
• 3. Xanthoria parietina.
• 4. Crustaceous species.

Parmelia prolixa is the most abundant of the Parmelias: it covers large spaces of the rocks and frequently competes for room with the Ramalinas, or in other areas with Physcia aquila and Lecanora parella.

A number of crustaceous species which form the sub-vegetation of the Ramalina belt, and also on the same level, clothe the steeper rock faces where shelter and moisture are insufficient to support the foliose forms. “In general the sub-vegetation of the eastern and northern coasts is largely composed of species that are common in Alpine and upland regions. This is due to the steepness of the rocks and also to the colder and drier conditions prevailing on these coasts.” An association of Rhizocarpon geographicum, Lecanora (sordida) glaucoma and Pertusaria concreta f. Westringii forms an almost continuous covering in some places, descending nearly to sea-level.

On sunnier and moister rocks with a south and south-west aspect the association is of more lowland forms such as Buellia colludens, B. stellulata, Lecanora smaragdula and L. simplex f. strepsodina.

(2) The Orange belt. “Below the Ramalinas, and between them and the sea, several deep yellow or orange-coloured lichens form a belt of varying width all round the coast. In summer, the colour of these lichens is so brilliant that the belt is easily recognized from a considerable distance.” The most abundant species occur mainly in the following order descending towards the sea:

• 1. Xanthoria parietina.
• 2. Placodium murorum.
• 3. Placodium tegularis.
• 4. Placodium decipiens.
• 5. Placodium lobulatum.

“On the stones and low shore rocks that lie just above the ordinary high-tide level Placodium lobulatum grows abundantly, covering the rocks with a continuous sheet of brilliant colour.” With these brightly coloured lichens are associated several with greyish thalli such as:

• Lecanora prosechoides.
• Lecanora umbrina.
• Lecanora Hageni.
• Rhizocarpon alboatrum.
• Biatorina lenticularis.
• Rinodina exigua var. demissa.
• Opegrapha calcarea f. heteromorpha.

(3) The Lichina vegetation, and (4) The Verrucaria maura belt. These two communities are intermingled, and it will therefore be better to consider them together. There are only two species of Lichina on this or any other shore, L. pygmaea and L. confinis; the latter grows above the tide-level, and sometimes high up on the cliffs, where it is subject to only occasional showers of spray: it forms on the Howth coast a band of vegetation four to five inches wide above the Verrucaria belt. Lichina pygmaea occurs nearer the water, and therefore mixed with and below Verrucaria maura. Those three zones were first pointed out by Nylander[1189] at Pornic, where however they were all submerged at high tide.

Verrucaria maura is one of the most abundant lichens of our rocky coasts, and is reported from Spitzbergen in the North to Graham Land in the Antarctic. It grows well within the range of sea-spray, covering great stretches of boulders and rocks with its dull-black crustaceous thallus. At Howth it is submerged only by the highest spring tides. Though it is the dominant lichen on that beach, other species such as V. memnonia, V. prominula, and V. aquatilis form part of the association, and more rarely V. scotina along with Arthopyrenia halodytes, A. leptotera and A. halizoa.

(5) The belt of marine Verrucarias. This association includes the species that are submerged by the tide for a longer or shorter period each day. The dominant species are Verrucaria microspora, V. striatula and V. mucosa. Arthopyrenia halodytes is also abundant; A. halizoa and A. marina are more rarely represented. Among the plants of Fucus spiralis, Verrucaria mucosa, the most wide-spreading of these marine forms, is “very conspicuous as a dark-green, almost black, band of greasy appearance stretching along the shore.” When growing in the shade, the thallus is of a brighter green colour.

An examination[1190] of the west coast of Ireland yielded much the same results, but with a still higher “white belt” formed mainly of Lecanora parella and L. atra which covered the rocks lying above high-water mark, “giving them the appearance of having been whitewashed.” A more general association for the same position as regards the tide is given by Wheldon and Wilson[1191] on the coasts of Arran as:

• Physcia aquila.
• Xanthoria parietina.
• Lecanora parella.
• Lecanora atra.
• Lecanora campestris.
• Placodium ferrugineum var. festivum.
• Placodium tegularis.
• Ramalina cuspidata.
• Physcia stellaris.
• Physcia tenella.
• Verrucaria maura.

A somewhat similar series of “formations” was determined by Sandstede[1192] on the coast of Rügen. On erratic granite boulders washed by the tide he found:

• Verrucaria maura.
• Lichina confinis.
• Lecanora prosechoides.
• Placodium lobulatum.

While in a higher position on similar boulders:

• Lecanora exigua.
• Lecanora dispersa.
• Lecanora galactina.
• Lecanora sulphurea.
• Lecanora saxicola.
• Lecanora caesiocinerea.
• Lecanora gibbosa.
• Lecanora atra.
• Lecanora parella.
• Lecidea colludens.
• Lecidea lavata.
• Lecidea nigroclavata f. lenticularis.
• Xanthoria parietina and f. aureola.
• Physcia subobscura.
• Physcia caesia.

And more rarely a few species of Lecidea.

b. Lichens of Sand-dunes. These lichens might be included with those of the terricolous communities, but they really represent a maritime community of xerophytic type, subject to the influence of salt spray but not within reach of the tide. They are sun-lichens and react to the strong light in the deeper colour of the thallus. In such a sun-baked area at Findhorn a luxuriant association of lichens was observed growing among short grass and plant debris. It consisted chiefly of:

• Parmelia physodes.
• Evernia prunastri.
• Cetraria aculeata.
• Cladonia cervicornis.
• Cladonia endiviaefolia.
• Peltigera spp.

On very arid situations the species of Cladonia are those that have a well-developed rather thick primary thallus, probably because such a thallus is able to retain moisture for a prolonged period[1193]. On shifting sand, as in the desert, there are no lichens; it is only on surfaces more or less fixed by marram grass that lichens begin to develop, though in the cool damp weather of autumn and winter, as observed by Wheldon and Wilson[1194], certain species associated with Myxophyceae, such as Collemaceae, may make their appearance, among others Leptogium scotinum, Collemodium turgidum and Collema ceranoides. Watson[1195] makes the same observation in his study of sand-dunes.

When the loose sand on the dunes of South Lancashire becomes cemented by algae and mosses several rare Lecideae are to be found on the decaying vegetation, and with further accumulation of humus Cladoniae appear and spread rapidly along with several species of Peltigera and the ubiquitous Parmelia physodes. The latter starts on dead twigs of Salix repens and spreads on to the surrounding soil where it forms patches some inches in diameter. The association also includes Lecidea uliginosa and Bilimbia sphaeroides.

On the more inland portions of the dunes numerous rather poorly developed Cladoniae and Cetraria aculeata were associated, while on the sides of “slacks” or “dune-pans” Collema pulposum, Cladonia sylvatica and several crustaceous lichens covered the soil. The wetter parts of the dunes were not found to be favourable to lichen growth.

Sandstede[1196] found on the sandy shores of Rügen, from the shore upwards: first a stretch of bare sand, then a few dune grasses with scattered scraps of Cladoniae, Peltigerae and Cetraria aculeata. Next in order sandbanks with Parmelia physodes, Cladonia sylvatica, Cl. alcicornis and Stereocaulon paschale. All these are species that occur on similar shores in the British Islands. Sandstede adds an extensive list of maritime species observed by him in Rügen.

A very careful tabulation of lichens at Blakeney Point in Norfolk was made by McLean[1197] and the table on p. 386 is reproduced from his paper. Sand, he writes, is present in all the associations and the presence or absence of stones marks the great difference between the two formations determined by dune and shingle.

(1) Bare sand, which is the first association listed, is an area practically without phanerogams; the few lichen plants, Cladonia furcata and Cetraria aculeata f. acanthella, are attached by slight embedding in the soil.

(2) Grey dune. The sand-loving lichens of the association grow in company with Hypnum cupressiforme and attain their greatest development. Other species which also occur there are Parmelia physodes and Evernia prunastri var. stictocera.

(3) Derelict dune. This part of the dune formation occurs here and there on the seaward margin where the grey dune has been worn down by the wind. It is more shingly, hence the presence of stone lichens; dune phanerogams are interspersed and with them a few fruticose lichens, such as Cladonia furcata.

(4) High shingle. The term indicates shingle aggregated into banks lying well above all except the highest tides. A large percentage of sand may be mixed with the stones and if no humus is present and the stones of small size, lichens may be absent altogether. Those occurring in the “loose shingle” are saxicolous. In the “bound shingle” where there is no grass the stones, fixed in a mixture of sand and humus, are well covered with lichens. With the presence of grass, a thin layer of humus covers the stones and a dense lichen vegetation is developed both of shingle and of dune species.

(5) Low shingle. This last association lies in the hollows among plants of Suacda fruticosa. Stability is high and tidal immersions regular and frequent. The dominant factor of the association is the quantity of humus and mud deposited around and over the stones. The lichens cover almost every available spot on the firmly embedded pebbles. The characteristic species of such areas are Lecanora badia and L. (Placodium) citrina which effect the primary colonization. To these succeed Lecanora atra and Xanthoria parietina. In time the mud overwhelms and partly destroys the lichens, so that the phase of luxuriant growth is only temporary.

Lecanora badia is conspicuously abundant at the sand end of this formation. Lecanora (Placodium) citrina disappears as the mud is left behind. Collema spp. also occur frequently on the mixture of mud and sand round the stones. The species on “low shingle” are those most tolerant of submersion: Verrucaria maura is confined to this area, where it is covered by the tide several hours each day.

McLean adds that Xanthoria parietina in its virescent form on Suaeda fruticosa also endures constant immersion; Lecanora badia does not occur above the tidal line and Lecanora galactina does not descend below tidal limits; the latter is an arenicolous species and colonizes some of the loosest and sandiest areas of shingle. Rhizocarpon confervoides is ubiquitous.

c. Mountain Lichens. On the mountain summits of our own and other lands are to be found lichens very similar to those of the far North the climatic conditions being the chief factors of importance in determining the formations. These regions are occupied by what Wheldon and Wilson[1198] describe as “a zone of Arctic-Alpine vegetation,” and they have recorded a series of lichen associations belonging to that zone on the schistose summits of the Perthshire mountains. The following is one of the most typical:

• Euopsis granatina.
• Sphaerophorus coralloides.
• Sphaerophorus fragilis.
• Gyrophora polyphylla.
• Cetraria tristis.
• Cetraria nivalis.
• Lecanora tartarea var. frigida.
• Lecanora upsaliensis.
• Aspicilia oculata.
• Pertusaria dactylina.
• Pertusaria glomerata.
• Stereocaulon denudatum.
• Parmelia saxatilis.
• Parmelia omphalodes.
• Parmelia lanata.
• Parmelia stygia.
• Stereocaulon tomentosum.
• Stereocaulon alpinum.
• Cladonia coccinca.
• Cladonia gracilis.
• Cladonia uncialis.
• Cladonia destricta.
• Cladonia racemosa.
• Lecidea arctica.
• Parmelia alpicola.
• Cetraria aculeata.
• Cetraria crispa.
• Cetraria islandica.
• Lecidea limosa.
• Lecidea alpestris.
• Lecidea demissa.
• Lecidea uliginosa.
• Lecidea cuprea.
• Lecidea Berengeriana.
• Lecidea cupreiformis.
• Lecidea atrofusca.

Again on the summit of Ben-y-Gloe the same authors[1199] have recorded Gyrophora erosa, G. torrefacta and G. cylindrica, Parmelia alpicola, Lecanora tartarea var. frigida, Lecidea limosa and L. arctica, the last two lichens thriving in the most bleak and exposed situations. Cladonia cervicornis grew in reduced squamulose cushions; Stereocaulon and Sphaerophorus in very compact forms, the outer stalks prostrate, the next inclined, the central ones erect so that points only are exposed and no lateral stress is caused by wind storms. Erect fruticose lichens are absent in this region, being represented only by Parmelia lanata, a semi-decumbent plant, and by Thamnolia vermicularis which is prostrate on the ground except where the points of the stalks turn up to catch the dew. Many of the Lecideae were observed to have large fruits and very little thallus: “the hyphae ramify in the minute interstices of the stone and the gonidia cluster under the lea of the apothecia: this is especially the case on loose stones where conditions are extremely dry.”

On the Continent an interesting study of the lichens of high altitudes was made by Maheu[1200] in the Savoyard Oberland. On the Great Casse at a height of 3861 m. he collected four mosses and sixteen lichens. These were:

• Stereocaulon condensatum.
• Gyrophora cylindrica.
• Gyrophora spodochroa.
• Solorina crocea.
• Solorina saccata.
• Parmelia encausta.
• Candelaria concolor.
• Caloplaca pyracea var. nivalis.
• Haematomma ventosum.
• Acarospora smaragdula.
• Psora decipiens.
• Buellia discolor.
• Buellia stellulata.
• Lecidea contigua var. steriza.
• Lecidea confluens.
• Dermatocarpon hepaticum.

He found that as he climbed higher and higher foliaceous species became rarer and crustaceous more abundant. The colour of the lichens on the high summits was slightly weakened and the thallus often reduced, but all were fertile and the apothecia normal and sporiferous. Lichens at less high altitudes where they emerge from the snow covering for longer periods and enjoy light and sunshine are, as already observed, often very brightly coloured and of luxuriant growth.

d. Tundra Lichens. In phyto-geography the term “tundra” is given to great stretches of country practically treeless and unsheltered within the Polar climate; the tundra extends from the zone of dwarfed trees on to the permanent ice or snow fields. The vegetation includes a few dwarfed trees, shrubs, etc., but is mainly composed of mosses and lichens; the latter being the most abundant. These are true climatic lichen formations.

Leighton[1201], in describing lichens from Arctic America brought home by the traveller, Sir John Richardson, quotes from the latter that: “the terrestrial lichens were gathered on Great Bear, and Great Slave Lakes before starting on our summer voyages after the snow had melted.... The barren grounds are densely covered for many hundreds of miles with Corniculariae and Cetrariae, and where the ground is moist with Cladoniae, while the boulders thickly scattered over the surface are clothed with Gyrophorae.... The smaller stones on the gravelly ridges of the Barren Grounds are covered with lichens.”

The accounts of tundra lichens that have been given by various travellers deal chiefly with the more prominent terricolous forms. They have been classified as “Cladina tundra,” including Cladonia rangiferina and Sphaerophorus coralloides, “Cetraria tundra,” and “Alectoria heath,” the latter the hardiest of all. Great swards of these lichens often alternate with naked stony soil.

Kihlman[1202] has noted, as characteristic of tundra formations, the compact cushion-like growth of the mosses which are thus enabled to store up water and to conduct it by capillarity throughout the mass to the highest stalks. Certain tundra lichens take on the same growth character as adaptations to the strenuous life conditions. Cetraria glauca f. spadicea with f. congesta and C. crispa are examples of this compact growth: they form a soft thick carpet of a yellowish-grey colour. Cladoniae also grow in crowded tufts, but are generally to be found in the more sheltered positions, in valleys between the tundra hills and in the clefts of the rocks, or between great boulders and stones where there is also more moisture.

The same kinds of lichens occur all over these northern regions. Birger Nilson[1203] gives as the principal earth-lichens in Swedish Lappland, Alectoria ochroleuca, A. nigricans, Cetraria nivalis, C. cucullata, Cladonia uncialis, Thamnolia (Cerania) vermicularis and Sphaerophorus coralloides.

Darbishire[1204] speaks of the extensive beds of various species of Cetraria in Ellesmere Land and King Oscar Land. Alectoria nigricans and A. ochrolenca were often found in pure communities, but even more frequently in close company with mosses. Though these fruticose lichens are not represented by many species in Arctic regions, they cover a very extensive area and form a very important feature in the vegetation.

Crustaceous lichens are not wanting: Lecanora tartarea f. frigida, L. epibryon and others are to be found in great sheets covering the mosses or the soil, or spreading over the stones and boulders. Cold has no deterrent effect, and their advance is only checked by the presence of perpetual snow.

e. Desert Lichens. The reduced rainfall of desert countries is unfavourable to general lichen growth and only the more xerophytic species—those with a stout cortex—can flourish in the adverse conditions of excessive light and dryness. Lichens, however, there are, in great numbers as far as individuals are concerned, though the variety is not great. The abundance of the crustaceous Lecanora esculenta in the deserts of Asia has already been noted. Flagey[1205] found it one of the dominant species at Biskra in the Sahara where it grows on the rocks. Patouillard[1206] in describing the flora of Tunis speaks of the great patches (societies) of Lecanora crassa f. deserti which at a distance look like milk spilled on the ground, or if growing on unequal surfaces take the aspect of plaster that has been passed over by some wheeled vehicle. At Biskra species of Heppia grow on the sand. Steiner[1207] also records the frequency of Heppia and of Endocarpon in the Sahara as well as of Gloeolichens which, as they are associated with gelatinous blue-green algae, can endure extreme and long-continued desiccation. These lichens, however, only form communities in clefts among the rocks where these abut on the desert. In the great plains the sand is too mobile and too often shifted by the sirocco to enable them to settle.

Bruce Fink[1208] discusses desert lichens and their adaptive characters: crustaceous species with a stout cortex are best able to withstand the long dry periods; conspicuously lobed thalli are lacking, as are lichens with fruticose structure though he thinks the latter are prevented from developing by the exposure to high winds and driving sand storms. Herre’s[1209] study of the desert lichen flora at Reno, Nevada, is full of interest. The district is situated at an altitude of 4500 feet east of the Sierra Nevada Mountains. The annual rainfall averages 8·21 inches, and a large part falls as snow during the winter months or as early spring rain. The summer is hot and dry and the diurnal changes of temperature are very great. Strong drying winds from the west or north are frequent.

At 5000 feet and upwards lichens are, in general, exceedingly abundant on all rock substrata and represent 57 species or subspecies, only three of these being arboreal: Buellia triphragmia occurs rarely, Xanthoria polycarpa is frequent on sage brush, while Candelariella cerinella though a rock-lichen grows occasionally on the same substratum. Caloplaca (Placodium) elegans is one of the most successful and abundant species and along with Lecanora (nine forms), Acarospora (seven forms) and Lecidia (five forms) comprises three-fourths of the rock surface occupied by lichens. The addition of Rinodina with two species and Gyrophora with four brings the computation of individuals in these desert rock formations up to nine-tenths of the whole. As the desert rocks pass to the Alpine, Gyrophora becomes easily the dominant genus followed by Acarospora, Caloplaca and Lecidea.

“The colouring characteristic of the rock ledges of the desert and cañon walls is often entirely due to lichens, and in a general way they form the only brilliant plant formations in a landscape notable for its subdued pale monotonous tones. Most conspicuous are Acarospora chlorophana and Caloplaca elegans, which form striking landmarks when covering great crags and rock walls. The next most conspicuous lichens are Rinodina oreina and Lecanora rubina and its allies, which often entirely cover immense boulders and northerly sloping rock walls.” Herre concludes that though desert conditions are unfavourable to most species of lichens, yet some are perfectly at home there and the rocks are just as thickly covered as in regions of greater humidity and less sunshine.

f. Aquatic Lichens. There is only one of the larger lichens that has acquired a purely aquatic habit, Hydrothyria venosa, a North American plant. It grows on rocks[1210] in the beds of streams, covering them often with a thick felt; it is attached at the base and the rather narrow fronds float freely in the current. The gonidium is Nostoc sp., and the thallus is of a bluish-grey colour; the fruits are small discoid reddish apothecia with an evanescent margin. It is closely allied to Peltigerae, some of which are moisture-loving though not truly aquatic.

The nearest approach to aquatic habit among the foliose forms in our country is Dermatocarpon aquaticum, with thick coriaceous rather contorted lobes; it inhabits rocks and stones in streams and lakes. Somewhat less continuously aquatic is D. miniatum var. complicatum which grows on damp rocks exposed to spray or occasionally to inundation. Lindsay[1211] has described it “on boulders by the side of the Tay, frequently covered by the river when flooded, and of a deep olive colour when under water”: both these lichens have a wide distribution in Europe, Africa, America and New Zealand.

In a discussion of lake shore plants Conway Macmillan[1212] describes on the flat shores a Dermatocarpon zone on the wet area nearest the lake, behind that a Biatora zone and further landward a Cladonia zone. On rounded rocky shores the same zones followed each other but were less broad: they were so close together that the Cladoniae, which with Stereocaulon paschale grow in profusion on all such shores, occurred within a couple of feet of the high-water mark.

M. C. Knowles[1213] reports concerning the lichen flora of some mountain lakes in Waterford, that a band of Dermatocarpon miniatum var. complicatum six feet wide grew all the way round the lakes between the winter and summer level of the water. Below that zone D. aquaticum formed another belt mingled with the moss Fontinalis and several species of crustaceous lichens Staurotheleae, Polyblastiae, etc.

Bruce Fink[1214] gives as a typical “amphibious angiocarpous lichen formation” of wet rocks in Minnesota: Dermatocarpon aquaticum, D. miniatum var. complicatum, Staurothele clopima and Verrucaria viridula. These “formations,” he says, “may be seen complete in places along the shores of Vermillion Lake and less well represented at other portions of the lake shore.” Macmillan found that on the rocky shores of Lake Superior the Dermatocarpon zone also occurred nearest the water.

Species with closed fruits such as Pyrenolichens, or with apothecia deeply sunk in the thallus and thus also well protected, seem to be best adapted to the aquatic life. Such in our own country are Lecanora lacustris, Bacidia inundata and others, with a number of Verrucariae: V. aethiobola, V. hydrela, V. margacea, etc.

Lettau[1215] gives as “formations” on rocks or boulders in the beds of streams in Thuringia:

• Verrucaria aethiobola.
• Verrucaria hydrela.
• Dermatocarpon aquaticum.
• Bacidia inundata.
• Lecanora aquatica.

In their ecological study of Perthshire lichens Wheldon and Wilson[1216] give two “formations.” The first is on rocks submerged for long periods, though in dry weather the lichens may be exposed, and can withstand desiccation for a considerable time:

• Pterygium Kenmorensis.
• Collema fluviatile.
• Lecanora lacustris.
• Lecanora epulotica.
• Bacidia inundata.
• Rhizocarpum obscuration.
• Rhizocarpum petraeum.
• Lecidea contigua.
• Lecidea albocoerulescens.
• Dermatocarpon miniatum var. complicatum.
• Dermatocarpon aquaticum.
• Verrucaria laevata.
• Verrucaria aethiobola.
• Verrucaria margacea.

The second group of species usually inhabits damp, shaded rocks of ravines or large boulders by streams or near waterfalls. It includes species of Collema, Sticta, Peltigera, Solorina, Pannaria, etc., with Opegrapha zonata, Porina lectissima and Verrucaria nigrescens.

The last-mentioned lichen grows by preference on limestone, but in excessive moisture[1217], as by the sea-side, the substratum seems to be of minor importance.

D. Lichens As Pioneers

a. Soil-formers. The part played by lichens in the “Economy of Nature” is of very real importance: to them is allotted the pioneer work of breaking down the hard rock surfaces and preparing a soil on which more highly developed plants can grow. This was pointed out by Linnaeus[1218] who thus describes the succession of plants: “Crustaceous lichens,” he writes, “are the first foundation of vegetation. Though hitherto we have considered theirs a trifling place among plants, nevertheless they are of great importance at that first stage in the economy of nature. When the rocks emerge from the seas, they are so polished by the force of the waves, that scarcely any kind of plant could settle on them, seen more especially near the sea. But very soon, in truth, the smallest crustaceous lichens begin to cover those arid rocks, and are sustained by minute quantities of soil and by imperceptible particles brought to them by rain and by the atmosphere. These lichens in time become converted by decay into a thin layer of humus, so that at length imbricate lichens are able to thrust their rhizoids into it. As these in turn change to humus by natural decay, various mosses such as Hypnum, Bryum and Polytrichum follow, and find suitable place and nourishment. In time there is produced by the dying down of the mosses such a quantity of soil that herbs and shrubs are able to establish themselves and maintain their existence.”

Similar observations have been made since Linnaeus’s day, among others by Guembel[1219] in his account of Lecanora ventosa. Either by the excretion of carbon dioxide which acidifies the surrounding moisture, or by the mechanical action of hyphae and rhizinae, the component particles of rocks such as granite are gradually dissolved and broken up. Rocks exposed to weather alone are unchanged, while those covered with lichens have their surface disintegrated and destroyed.

The decaying parts of the lichen thallus add to the soil material as observed by Linnaeus, and in time mosses follow, and, later, phanerogams. Goeppert[1220] has pointed out the succession observed on roofs of houses as: “first some lichen such as Lecanora saxicola, then the moss Grimmia pulvinata, which forms compact cushions on which later grow Poa compressa, small crucifers, etc.”

Goeppert[1220] has noted as special rock-destroyers some foliaceous species, Parmelia saxatilis, P. stygia and P. encausta, the underlying rock being roughened and broken up by their rhizoids. Species of Gyrophora and Sphaerophorus have the same disintegrating effect, so that the surface of the rock may in time lose its coherence to a depth of 2 to 4 inches. Crustaceous species such as Lecanora polytropa, Candelariella vitellina, etc., exercise an equally powerful solvent action, while underneath closely appressed growers like Lecanora atra and Acarospora smaragdula the stone is converted to a friable substance that can be sliced away with a knife.

Salter[1221] concluded that oxalic acid was the principal agent in disintegration. He found that it acted more or less rapidly on minerals and almost any class of saline compounds; it even attacked glass finely powdered, though silica remained unchanged.

Bachmann[1222] found that granite was reduced by lichens to a clay-like granular yellow mass in a comparatively short time, the lichen seizing on the particles of mica first; but the spread of the lichen over the rock, he observes, is largely directed by the amount of humidity and by the chance of gaining a foothold. In the case of calcareous rocks he[1223] tested the relative dampness of those containing lichens and those that were lichen-free. In the former case water was absorbed more freely and retained much longer than in the barren rock, thus encouraging further vegetation.

Lucy E. Braun[1224] has described the successive colonization of limestone conglomerate in Cincinnati. The rock is somewhat resistant to erosion and stands out in irregular outcrops on the hillsides of the region. The first plants to gain a footing are certain crustaceous lichens, Lecidea sp., Pertusaria communis, Staurothele umbrina, Verrucaria muralis and Placodium citrinum which occur as patches on the smoother and more exposed rock faces. With these were associated small quantities of a moss, Grimmia apocarpa. In the second stage of growth Dermatocarpon miniatum, and, to a lesser degree, a gelatinous Omphalaria sp. were the most prominent plants, but mosses were more in evidence, and the next stage consisted almost exclusively of mosses and hepatics with Peltigera canina. A thick layer of humus was gradually built up by these plants on which Phanerogamous plants were able to flourish.

In tropical countries the first vegetation to settle on bare rocks would seem to be blue-green gelatinous algae. Three years after the eruption of Krakatoa, dark-green layers of these plants were found by Treub[1225] on the surface of the pumice and ash, and on the loose stones in the ravines of the mountain. It was only at a later stage that lichens appeared.

b. Outposts of vegetation. Lichens are the only plants that can survive extreme conditions of cold or of heat. They grow in Polar regions where no other vegetation could obtain sustenance; they are to be found at great heights on mountains all over the globe; and, on arid desert rocks they persist through long dry seasons, depending almost entirely on night dews for the supply of moisture. Here we have true lichen formations in the sense of modern ecology.

CHAPTER X
ECONOMIC AND TECHNICAL

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.