C. Lichen Communities

Lichen communities fall into a few definite groups, though, as we shall see, not a few species may be found to occur in several groups—species that have been designated by some workers as “wanderers.” The leading communities are:

1. Arboreal, including those that grow on leaves, bark or wood.

2. Terricolous, ground-lichens.

3. Saxicolous, rock-lichens.

4. Omnicolous, lichens that can exist on the most varied substrata, such as bones, leather, iron, etc.

5. Localized Communities in which owing to special conditions the lichens may become permanent and dominant.

In all the groups lichens are more or less abundant. In arboreal and terricolous formations they may be associated with other plants; in saxicolous and omnicolous formations they are the dominant vegetation. It will be desirable to select only a few of the typical communities that have been observed and recorded by workers in various lands.

1. ARBOREAL

Arboreal communities may be held to comprise those lichens that grow on wood, bark or leaves. They are usually the dominant and often the sole vegetation, but in some localities there may be a considerable development of mosses, etc., or a mantle of protococcaceous algae may cover the bark. Certain lichens that are normally corticolous may also be found on dead wood or may be erratic on neighbouring rocks: Usnea florida for instance is a true corticolous species, but it grows occasionally on rocks or boulders generally in crowded association with other foliose or fruticose lichens.

Most of the larger lichens are arboreal, though there are many exceptions: Parmelia perlata develops to a large size on boulders as well as on trees; some species of Ramalinae are constantly saxicolous while there are only rare instances of Roccellae that grow on trees. The purely tropical or subtropical genera are corticolous rather than saxicolous, but species that have appeared in colder regions may have acquired the saxicolous habit: thus Coenogonium in the tropics grows on trees, but the European species, C. ebeneum, grows on stone.

a. Epiphyllous. These grow on Ferns or on the coriaceous leaves of evergreens in the tropics. Many of them are associated with Phycopeltis, Phyllactidium or Mycoidea, and follow in the wake of these algae. Observations are lacking as to the associations or societies of these lichens whether they grow singly or in companies. The best known are the Strigulaceae: there are six genera in that family, and some of the species have a wide distribution. The most frequent genus is Strigula associated with Phycopeltis which forms round grey spots on leaves, and is almost entirely confined to tropical regions. Chodat[1134] records a sterile species, S. Buxi, on box leaves from the neighbourhood of Geneva.

Other genera, such as those of Ectolechiaceae, which inhabit fern scales and evergreen leaves, are associated with Protococcaceae. Pilocarpon leucoblepharum with similar gonidia grows round the base of pine-needles. It is found in the Caucasus. In our own woods, along the outer edges, the lower spreading branches of the fir-trees are often decked with numerous plants of Parmelia physodes, a true “plant society,” but that lichen is a confirmed “wanderer.” Biatorina Bouteillei, on box leaves, is a British and Continental lichen.

b. Corticolous. In this series are to be found many varying groups, the type of lichen depending more on the physical nature of the bark than on the kind of trees. Those with a smooth bark such as hazel, beech, lime, etc., and younger trees in general, bear only crustaceous species, many of them with a very thin thallus, often partly immersed below the surface. As the trees become older and the bark takes on a more ragged character, other types of lichens gain a foothold, such as the thicker crustaceous forms like Pertusaria, or the larger foliose and fruticose species. The moisture that is collected and retained by the rough bark is probably the important factor in the establishment of the thicker crusts, and, as regards the larger lichens, both rhizinae and hold-fasts are able to gain a secure grip of the broken-up unequal surface, such as would be quite impossible on trees with smooth bark.

Among the first to pay attention to the ecological grouping of corticolous lichens was A.L. Fée[1135], a Professor of Natural Science and an Army doctor, who wrote on many literary and botanical subjects. In his account of the Cryptogams that grow on “officinal bark,” he states that the most lichenized of all the Cinchonae was the one known as “Loxa,” the bark of which was covered with species of Parmelia, Sticta and Usnea along with crustaceous forms of Lecanora, Lecidea, Graphis and Verrucaria. Another species, Cinchona cordifolia, was completely covered, but with crustaceous forms only: species of Graphidaceae, Lecanora and Lecidea were abundant, but Trypethelium, Chiodecton, Pyrenula and Verrucaria were also represented. On each species of tree some particular lichen was generally dominant:

Fries[1136], in his geography of lichens, distinguished as arboreal and “hypophloeodal” species of Verrucariaceae, while the Graphideae, which also grew on bark, were erumpent. Usnea barbata, Evernia prunastri, etc., though growing normally on trees might, he says, be associated with rock species.

More extensive studies of habitat were made by Krempelhuber[1137] in his Bavarian Lichens. In summing up the various “formations” of lichens, he gives lists of those that grow, in that district, exclusively on either coniferous or deciduous trees, with added lists of those that grow on either type of tree indifferently. Among those found always on conifers or on coniferous wood are: Letharia vulpina, Cetraria Laureri, Parmelia aleurites and a number of crustaceous species. Those that are restricted to the trunks and branches of leafy trees are crustaceous with the exception of some foliose Collemaceae such as Leptogium Hildenbrandii, Collema nigrescens, etc.

Arnold[1138] carried to its furthest limit the method of arranging lichens ecologically, in his account of those plants from the neighbourhood of Munich. He gives “formation” lists, not only for particular substrata and in special situations, but he recapitulates the species that he found on the several different trees. It is not possible to reproduce such a detailed survey, which indeed only emphasizes the fact that the physical characters of the bark are the most important factors in lichen ecology: that on smooth bark, whether of young trees, or on bark that never becomes really rugged, there is a preponderance of species with a semi-immersed thallus, and very generally of those that are associated with Trentepohlia gonidia, such as Graphidaceae or Pyrenulaceae, though certain species of Lecidea, Lecanora and others also prefer the smooth substratum.

Bruce Fink[1139] has published a series of important papers on lichen communities in America, some of them similar to what we should find in the British Isles.

On trees with smooth bark he records in the Minnesota district:

On rough bark he records:

Finally, as generally representative of the commonest lichens in our woods of deciduous trees, including both smooth- and rough-barked, the community of oak-hazel woods as observed by Watson[1140] in Somerset may be quoted:

And on the soil of these woods:

Paulson[1141], from his observations of lichens in Hertfordshire, has concluded that the presence or absence of lichens on trees is influenced to a considerable degree by the nature of the soil. They were more abundant in woods on light well-drained soils than on similar communities of trees on heavier soils, though the shade in the former was slightly more dense and therefore less favourable to their development; the cause of this connection is not known.

c. Lignicolous. Lichens frequenting the branches of trees do not long continue when these have fallen to the ground. This may be due to the lack of light and air, but Bouly de Lesdain[1142] has suggested that the chemical reactions produced by the decomposition of the bast fibres are fatal to them, Lecidea parasema alone continuing to grow and even existing for some time on the detached shreds of bark.

On worked wood, such as old doors or old palings, light and air are well provided and there is often an abundant growth of lichens, many of which seem to prefer that substratum: the fibres of the wood loosened by weathering retain moisture and yield some nutriment to the lichen hyphae which burrow among them. Though a number of lichens grow willingly on dead wood, there are probably none that are wholly restricted to such a habitat. A few, such as the species of Coniocybe, are generally to be found on dead roots of trees or creeping loosely over dead twigs. They are shade lichens and fond of moisture.

The species on palings—or “dead wood communities”—most familiar to us in our country are:

These may be found in very varying association. It has indeed been remarked that the dominant plant may be simply the one that has first gained a footing, though the larger and more vigorous lichens tend to crowd out the others. Bruce Fink[1143] has recorded associations in Minnesota:

On wood:

On rotten stumps and prostrate logs: Peltigera canina, Cladonia fimbriata var. tubaeformis, Cl. gracilis, Cl. verticillata, Cl. symphicarpia, Cl. macilenta, Cl. cristatella.

Except for one or two species such as Buellia turgescens, Cladonia symphicarpia, etc., the associations could be easy paralleled in our own country, though with us Peltigera canina, Cladonia gracilis and Cl. verticillata are ground forms.

2. TERRICOLOUS

In this community other vegetation is dominant, lichens are subsidiary. In certain conditions, as on heaths, they gain a permanent footing, in others they are temporary denizens and are easily crowded out. As they are generally in close contact with the ground they are peculiarly dependent on the nature of the soil and the water content. There are several distinct substrata to be considered each with its characteristic flora. Cultivated soil and grass lands need scarcely be included, as in the former the processes of cultivation are too harassing for lichen growth, and only on the more permament somewhat damp mossy meadows do we get such a species as Peltigera canina in abundance. Some of the earth-lichens are among the quickest growers: the apothecia of Baeomyces roseus appear and disappear within a year. Thrombium epigaeum develops in half a year; Thelidium minutulum in cultures grew from spore to spore, according to Stahl[1144], in three months.

There are three principal types of soil composition: (1) that in which there is more or less of lime; (2) soils in which silica in some form or other predominates, and (3) soils which contain an appreciable amount of humus.

Communities restricted to certain soils such as sand-dunes, etc., are treated separately.

a. On calcareous soil. Any admixture of lime in the soil, either as chalk, limy clay or shell sand is at once reflected in the character of the lichen flora. On calcareous soil we may look for any of the squamulose Lecanorae or Lecideae that are terricolous species, such as Lecanora crassa, L. lentigera, Placodium fulgens, Lecidea lurida and L. decipiens. There are also the many lichens that grow on mortar or on the accumulated debris mixed with lime in the crevices of walls, such as Biatorina coeruleonigricans, species of Placodium, several species of Collema and of Verrucariaceae.

Bruce Fink[1145] found in N.W. Minnesota an association on exposed calcareous earth as follows:

This particular association occupied the slope of a hill that was washed by lime-impregnated water. It was normally a dry habitat and the lichens were distinguished by small closely adnate thalli.

There are more lichens confined to limy than to sandy soil. Arnold[1146] gives a list of those he observed near Munich on the former habitat:

It is interesting to note how many of these lichens specialized as to habitat are forms of species that grow in other situations.

b. On siliceous soil. Lichens are not generally denizens of cultivated soil; a few settle on clay or on sandbanks. Cladonia fimbriata and Cl. pyxidata grow frequently in such situations; others more or less confined to sandy or gravelly soil are, in the British Isles:

These very generally grow in extended societies of one species only.

In his enumeration of soil-lichens Arnold[1146] gives 40 species that grow on siliceous soil, as against 57 on calcareous. Many of them occurred on both. Those around Munich on siliceous soil only were:

Mayfield[1147] in his account of the Boulder Clay lichen flora of Suffolk found only four species that attained to full development on banks and hedgerows. These were: Collema pulposum, Cladonia pyxidata, Cl. furcata var. corymbosa and Peltigera polydactyla.

On bare heaths of gravelly soil in Epping Forest Paulson and Thompson[1148] describe an association of such lichens as:

And on flints in the soil: Lecidea crustulata and Rhizocarpon confervoides. They found that Peltigera spuria colonized very quickly the burnt patches of earth which are of frequent occurrence in Epping Forest, while on wet sandy heaths amongst heather they found associated Cladonia sylvatica f. tenuis and Cl. fimbriata subsp. fibula.

c. On bricks, etc. Closely allied with siliceous soil-lichens are those that form communities on bricks. As these when built into walls are more or less smeared with mortar, a mixture of lime-loving species also arrives. Roof tiles are more free from calcareous matter. Lesdain[1149] noted that on the dunes, though stray bricks were covered by algae, lichens rarely or never seemed to gain a footing.

There are many references in literature to lichens that live on tiles. A fairly representative list is given by Lettau[1150] of “tegulicolous” species.

Several of these are more or less calcicolous and others are wanderers, indifferent to the substratum. Though certain species form communities on bricks, tiles, etc., none of them is restricted to such artificial substrata.

d. On humus. Lichens are never found on loose humus, but rocks or stumps of trees covered with a thin layer of earth and humus are a favourite habitat, especially of Cladoniae. One such “formation” is given by Bruce Fink[1151] from N. Minnesota; with the exception of Cladonia cristatella, the species are British as well as American:

e. On peaty soil. Peat is generally found in most abundance in northern and upland regions, and is characteristic of mountain and moorland, though there are great moss-lands, barely above sea-level, even in our own country. Such soil is of an acid nature and attracts a special type of plant life. The lichens form no inconsiderable part of the flora, the most frequent species being members of the Cladoniaceae.

The principal crustaceous species on bare peaty soil in the British Isles are Lecidea uliginosa and L. granulosa. The former is not easily distinguishable from the soil as both thallus and apothecia are brownish black. The latter, which is often associated with it, has a lighter coloured thallus and apothecia that change from brick-red to dark brown or black; Wheldon and Wilson[1152] remarked that after the burning of the heath it was the first vegetation to appear and covered large spaces with its grey thallus. Another peat species is Icmadophila ericetorum, but it prefers damper localities than the two Lecideae.

To quote again from Arnold[1153]: 24 species were found on turf around Munich, 13 of which were Cladoniae, but only four species could be considered as exclusively peat-lichens. These were:

The last is a very rare lichen in Central Europe and is generally found on sandy soil. Arnold considered that near Munich, for various reasons, there was a very poor representation of turf-lichens.

f. On mosses. Very many lichens grow along with or over mosses, either on the ground, on rocks or on the bark of trees, doubtless owing to the moisture accumulated and retained by these plants. Besides Cladoniae the commonest “moss” species in the British Isles are Bilimbia sabulosa, Bacidia muscorum, Rinodina Conradi, Lecidea sanguineoatra, Pannaria brunnea, Psoroma hypnorum and Lecanora tartarea, with species of Collema and Leptogium and Diploschistes bryophilus.

Wheldon and Wilson[1154] have listed the lichens that they found in Perthshire on subalpine heath lands, on the ground, or on banks amongst mosses:

As already described Lecanora tartarea[1155] spreads freely over the mosses of the tundra. Aigret[1156] in a study of Cladoniae notes that Cl. pyxidata, var. neglecta chooses little cushions of acrocarpous mosses, which are particularly well adapted to retain water. Cl. digitata, Cl. flabelliformis and some others grow on the mosses which cover old logs or the bases of trees.

g. On fungi. Some of the fungi, such as Polyporei, are long lived, and of hard texture. On species of Lenzites in Lorraine, Kieffer[1157] has recorded 15 different forms, but they are such as naturally grow on wood and can scarcely rank as a separate association.

3. SAXICOLOUS

Lichens are the dominant plants of this and the following formations, they alone being able to live on bare rock; only when there has been formed a nidus of soil can other plants become established.

a. Characters of Mineral Substrata. It has been often observed that lichens are influenced not only by the chemical composition of the rocks on which they grow but also by the physical structure. Rocks that weather quickly are almost entirely bare of lichens: the breaking up of the surface giving no time for the formation either of thallus or fruit. Close-grained rocks such as quartzite have also a poor lichen flora, the rooting hyphae being unable to penetrate and catch hold. Other factors, such as incidence of light, and proximity of water, are of importance in determining the nature of the flora, even where the rocks are of similar formation.

b. Colonization on Rocks. When a rock surface is laid bare it becomes covered in time with lichens, and quite fresh surfaces are taken possession of preferably to weathered surfaces[1158]. The number of species is largest at first and the kind of lichen depends on the flora existing in the near neighbourhood. Link[1159], for instance, has stated that Lichen candelarius was the first lichen to appear on the rocks he observed, and, if trees were growing near, then Lichen parietinus and Lichen tenellus followed soon after. After a time the lichens change, the more slow-growing being crowded out by the more vigorous. Crustaceous species, according to Malinowski[1160], are most subject to this struggle for existence, and certain types from the nature of their thallus are more easily displaced than others. Those with a deeply cracked areolated thallus become disintegrated in the older central areas by repeated swelling and contracting of the areolae as they change from wet to dry conditions. Particles of the thallus are thus easily dislodged, and bare places are left, which in time are colonized again by the same lichen or by some invading species. There may result a bewildering mosaic of different thalli and fruits mingling together. Some forms such as Rhizocarpum geographicum which have a very close firm thallus do not break away. In the course of time lichen communities come and go, and the plants of one locality may be different from those of another for no apparent reason.

The question of colonization[1161] was studied by Bruce Fink[1162] on a “riprap” wall of quartz, 30 years old, built to protect and brace a railway in Iowa. Near by was a grass swamp which supplied moisture especially to the lower end of the wall. A few boulders were present in the vicinity, but the nearest lichen “society” was on trees about 150 metres away and these bore corticolous Parmelias, Physcias, Ramalinas, Placodiums, Lecanoras and Rinodines which were only very sparingly represented on the riprap. Moisture-loving species never gained a footing; the extreme xerophytic conditions were evidenced by the character of the lichens, Biatora myriocarpoides (Lecidea sylvicola) occupying the driest parts of the wall. Lower down where more moisture prevailed Bacidia inundata and Stereocaulon paschale were the dominant species. Some 30 species or forms were listed of which 11 were Cladonias that grew mainly on debris from the disintegration of the wall. With the exception of two or three species the number of individuals was very small.

Some of these lichens had doubtless come from the boulders, others from the trees; the Cladonias were all known to occur within a few miles, but most of the species had been wind-borne from some distance. The Stereocaulon present did not exist elsewhere in Iowa; it had evidently been brought by the railroad cars, possibly on telegraph poles.

A similar wall on the south side of the railway, subject to even more xerophytic conditions but with less disintegration of the surface, had a larger number of individuals though fewer species. Only one Cladonia and one Parmelia had gained a footing, the rest were crustaceous, Buellia myriocarpa being one of the most frequent.

There are two types of rock of extreme importance in lichen ecology: those mainly composed of lime (calcareous), and those in which silica or silicates preponderate (siliceous). They give foothold to two corresponding groups of lichen communities, calcicolous and silicicolous.

c. Calcicolous. The pioneer in this section of lichen ecology is H. F. Link, who was a Professor of Natural Science and Botany at Rostock, then at Breslau, and finally in Berlin. He[1163] published in 1789, while still at Rostock, an account of limestone plants in his neighbourhood, most of them being lichens. In a later work he continues his Botanical Geography or “Geology” and gives more precise details as to the plants, some of which are essentially calcicolous though many of them he records also on siliceous rocks.

Most calcicolous lichens are almost completely dependent on the lime substratum which evidently supplies some constituent that has become necessary to their healthy growth. Calcareous rocks are usually of softer texture than those mainly composed of silica, and not only the rhizoidal hyphae but the whole thallus—both hyphae and gonidia—may be deeply embedded. Only the fruits are visible and they are, in some species, lodged in tiny depressions (foveolae) scooped out of the surface by the lichen-acids acting on the easily dissolved lime.

Those obligate lime species may be found in associations on almost any calcareous rock. Watson[1164] has given us a list of species that inhabit carboniferous limestone in Britain. Wheldon and Wilson[1165] have described in West Lancashire the “grey calcareous rocks blotched with black patches of Pannarias (Placynthium nigrum) and Verrucarias, or dark gelatinous rosettes of Collemas. White and grey Lecanorae and Verrucariae spread extensively, some of them deeply pitting the surface. These more sombre or colourless species are enlivened by an intermixture of orange-yellow Physciae (Xanthoriae) and Placodii by the ochrey films of Lecanora ochracea and lemon-yellow of Lecanora xantholyta. Amongst the greenish scaly crusts of Lecanora crassa may be seen the bluish cushions of Lecidea coeruleonigricans, the whole forming an exquisite blend of tints.”

The flora recorded by Flagey[1166] on the cretaceous rocks of Algeria in the Province of Constantine does not greatly differ, some of the species being identical with those of our own country. Placodiums and Rinodinas were abundant, as also Lecanora calcarea, Acarospora percaenoides and Urceolaria actinostoma var. calcarea. Also a few Lecideae along with Verrucaria lecideoides, V. fuscella, V. calciseda and Endocarpon monstrosum. The rocks of that region are sometimes so covered with lichens that the stone is no longer visible.

Bruce Fink[1167] gives a typical community on limestone bluffs in Minnesota:

Forssell[1168] pointed out an interesting selective quality in the Gloeolichens which are associated with the gelatinous algae, Chroococcus, Gloeocapsa and Xanthocapsa. The genera containing the two former grow on siliceous rocks with the exception of Synalissa. The genera Omphalaria, Peccania, Anema, Psorotichia and Enchylium, in which Xanthocapsa is the gonidium, grow on calcareous rocks. Collemopsidium is the only Xanthocapsa associate that is silicicolous.

d. Silicicolous. There is greater variety in the mineral composition and in the nature of the surface in siliceous than in calcareous rocks; they are also more durable and give support to a large number of slow-growing forms.

Silicon enters into the composition of many different types, from the oldest volcanic to the most recent of sedimentary rocks. Some of these are of hard unyielding surface on which only a few lichens are able to attach themselves. Such a rock is instanced by Servit[1169] as occurring in Bohemia, and is known as Lydite or Lydian stone, a black flinty jasper. The association of lichens on this smooth rock was almost entirely Acarospora chlorophana and Rinodina oreina, which as we shall see occur again as a “desert” association in Nevada; these two lichens grow equally well in sun or shade, and either sheltered or exposed as regards wind and rain. Acarospora chlorophana, according to Malinowski[1170], arrives among the first on rocks newly laid bare, and forms large societies, though in time it gives place to Lecanora glaucoma (L. sordida), a common silicicolous lichen.

A difference has been pointed out by Bachmann[1171] between the lichens of acid and of basic rocks. The acid series, such as quartz- and granite-porphyry, contain 70 per cent. and more of oxide of silica; the basic—diabase and basalt—not nearly 50 per cent. He observed that Rhizocarpon geographicum was the most frequent lichen of the acid porphyry, while on basalt there were only small scattered patches. Pertusaria corallina was abundant only on granitic rocks. On the other hand Pertusaria lactea f. cinerascens, Diploschistes scruposus, D. bryophilus and Buellia leptocline preferred the basic substratum of diabase and basalt. In this case it is the chemical rather than the physical character of the rocks that affects the lichen flora, as porphyry and basalt are both close-grained, and are outwardly alike except in colouration.

Other rocks, such as granite, in which the different crystals, quartz, mica and felspar are of varying hardness, are favourite habitats as affording not only durability but a certain openness to the rhizoidal hyphae, though in Shetland, West[1172] found the granitic rocks bare owing to their too rapid weathering. In these rocks the softer basic constituents such as the mica are colonized first; the quartz remains a long time naked, though in time it also is covered. Wheldon and Wilson[1173] point out that the sandstone near to intrusive igneous rocks has become close-grained and indurated and bears Lecanora squamulosa, L. picea, Lecidea rivulosa and Rhizocarpon petraeum, which were not seen on the unaltered sandstone. It was also observed by Stahlecker[1174], that, in layered rocks, the lichen chose the surface at right angles to the layering as the hyphae thus gain an easier entrance.

It will only be possible to give a few typical associations from the many that have been published. Crustaceous forms are the most abundant.

On granite and on quartzite not disintegrated Malinowski[1175] listed:

On granite and quartzite disintegrated:

On these disintegrated rocks there is a constant struggle for existence between the various species; the victorious association finally consists of Lecanora badia, L. cenisia and Lecidea confluens with occasional growths of the following species:

A number of rock associations have been tabulated by Wheldon and Wilson[1176] for Perthshire. Among others they give some of the most typical lichens on granitic and eruptive rocks:

On siliceous rocks in West Lancashire the same authors[1177] depict the lichen flora as follows: “There are many grey Parmeliae and Cladoniae with coral-like Sphaerophorei on the rocks, and on the walls smoky-looking patches of Parmelia fuliginosa and ragged fringes of Platysma glaucum and Evernia furfuracea. On the higher scars, flat topped tabular blocks exhibit black scaly Gyrophoreae, dingy green Lecidea (Rhizocarpon) viridiatra and mouse-coloured L. rivulosa. Suborbicular (whitish) patches of Pertusaria lactea and P. dealbata enliven the general sadness of tone, and everywhere loose rocks and stones are covered with the greyish-black spotted thallus of Lecidea contigua.”

On the Silurian series of rocks in the same district they describe a somewhat brighter coloured flora: “First Stereocaulons invite attention, and greenish or yellowish shades are introduced by an abundance of Lecanora sulphurea, L. polytropa, Rhizocarpon geographicum and Parmelia conspersa, often beautifully commingled with grey species such as Lecidea contigua and L. stellulata, and reddish angular patches of Lecanora Dicksonii. Also an abundance of orbicular patches of Haematomma ventosum with its reddish-brown apothecia.” A brightly coloured association on the cretaceous sand-rocks of Saxon Switzerland has been described as “Sulphur lichens.” These have recently[1178] been determined as chiefly Lepraria chlorina, in less abundance Lecidea lucida and Calicium arenarium, with occasional growths of Coniocybe furfuracea and Calicium corynellum.

4. OMNICOLOUS LICHENS

Some account must be taken in any ecological survey of those lichens that are indifferent to substrata. Certain species have become so adapted to some special habitat that they never or rarely wander; others, on the contrary, are true vagabonds in the lichen kingdom and settle on any substance that affords a foothold: on leather, bones, iron, pottery, etc. There can be no sustenance drawn from these supports, or at most extremely little, and it is interesting to note in this connection that while some rock-lichens are changed to a rusty-red colour by the infiltration of iron—often from a water medium containing iron-salts—those that live directly on iron are unaffected.

The “wanderers” are more or less the same in every locality and they pass easily from one support to another. Bouly de Lesdain[1179] made a tabulation of such as he found growing on varied substances on the dunes round Dunkirk and they well represent these omnicolous communities. It is in such a no man’s land that one would expect to find an accumulation of derelict materials, not only favourably exposed to light and moisture, but undisturbed for long periods and bordering on normal lichen associations of soil, tree and stones. Arnold[1180] also noted many of these peculiar habitats.

The following were noted by Lesdain and other workers:

On iron—Xanthoria parietina, Physcia obscura and var. virella, Ph. ascendens, Placodium (flavescens) sympageum, Pl. pyraceum, Pl. citrinum, Candelariella vitellinum, Rinodina exigua, Lecanora campestris, L. umbrina, L. galactina, Lecania erysibe, Bacidia inundata. Xanthoria parietina is one of the commonest wandering species; it was found by Richard[1181] on an old cannon lying near water, that was exfoliated by rust.

On tar—Lecanora umbrina.

On charcoal—Rinodina exigua, Lecanora umbrina.

On bones—Xanthoria parietina, Physcia ascendens, Ph. tenella, Placodium citrinum, Pl. lacteum, Rinodina exigua, Lecanora galactina, L. dispersa, L. umbrina, Lecania erysibe, L. cyrtella, Acarospora pruinosa, A. Heppii, Bacidia inundata, B. muscorum, Verrucaria anceps, V. papillosa.

In Arctic regions in Ellesmere Land and King Oscar Land, Darbishire[1182] found on bones: Lecanora varia, L. Hageni, Rinodina turfacea and Buellia parasema (disciformis). He could not trace any effect of the lichens on the substratum.

On charcoal—Rinodina exigua, Lecanora umbrina.

On dross or clinkers—Parmelia dubia, Physcia obscura, Ph. ascendens f. tenella, Ph. pulverulenta, Xanthoria parietina, Placodium pyraceum, Pl. citrinum, Rinodina exigua, Lecanora dispersa, L. umbrina, Lecania erysibe.

On glass[1183]—Physcia ascendens f. tenella, Buellia canescens. Richard has recorded the same lichens on the broken glass of walls and in addition: Xanthoria parietina, Lecanora crenulata, L. dispersa, Lecania erysibe, Rinodina exigua, and Buellia canescens.

On earthenware, china, etc.—Physcia ascendens f. tenella, Lecanora umbrina, L. dispersa, Lecania (? Biatorina) cyrtella, Verrucaria papillosa, Bacidia inundata.

On leather—Nearly fifty species or varieties were found by Lesdain on old leather on the dunes. Cladonias, Parmelias and Physcias were well represented with one Evernia and a large series of crustaceous forms. He adds a note that leather is an excellent substratum: lichens covered most of the pieces astray on the dunes. Similar records have been made in Epping Forest by Paulson and Thompson[1184] who found Cladonia fimbriata var. tubaeformis and Lecidea granulosa growing on an old boot. These authors connect the sodden condition of the leather with its attraction for lichens.

On pasteboard—Even on such a transient substance as this Lesdain found a number of forms, most of them, however, but poorly developed: Cladonia furcata (thallus), Parmelia subaurifera (beginning), Xanthoria parietina (beginning), Physcia obscura, Placodium citrinum (thallus), Pl. pyraceum, Lecanora umbrina, Bacidia inundata and Polyblastia Vouauxi var. charticola.

On linoleum—Xanthoria parietina, Physcia ascendens f. tenella, Rinodina exigua, Lecanora umbrina.

On indiarubber—Physcia ascendens f. tenella.

On tarred cloth—Xanthoria parietina, Placodium citrinum, Pl. pyraceum, Rinodina exigua, Lecanora umbrina, Lecania erysibe, Bacidia inundata.

On felt—Bacidia inundata, B. muscorum.

On cloth (cotton, etc.)—Bacidia inundata.

On silk—Physcia ascendens, Ph. obscura, Placodium citrinum (thallus), Lecanora umbrina, Bacidia inundata.

On cord—Physcia ascendens f. tenella, Placodium citrinum (thallus).

On excreta—One would scarcely expect to find lichens on animal droppings, but as some of these harden and lie exposed for a considerable time, some quick-growing species attain to more or less development on what is, in any case, an extremely favourable habitat for fungi and for many minute organisms. Paulson and Thompson found tiny fruiting individuals of Cladonia macilenta and Cl. fimbriata var. tubaeformis growing on the dry dung of rabbits in Epping Forest. On the same type of pellets Lesdain records Physcia ascendens f. leptalea, Cladonia pyxidata, Bacidia inundata and B. muscorum; and on sheep pellets: Physcia ascendens f. leptalea and Placodium citrinum; while on droppings of musk-ox in Ellesmere Land Darbishire found Biatorina globulosa, Placodium pyraceum, Gyalolechia subsimilis, Lecanora epibryon, L. verrucosa, Rinodina turfacea and even, firmly attached, Thamnolia vermicularis.

It would be difficult to estimate the age of these lichens, but it seems evident that the “wanderers” are all more or less quick growers, and the lists also prove conclusively their complete indifference to the substratum, as the same species occur again and again on the very varied substances.

5. LOCALIZED COMMUNITIES

Lichens may be grouped ecologically under other conditions than those of substratum. They respond very readily to special environments, and associations arise either of species also met with elsewhere, or of species restricted to one type of surroundings. Such associations or communities might be multiplied indefinitely, but only a few of the outstanding ones will be touched on.

a. Maritime Lichens. This community is the most specialized of any, many of the lichens having become exclusively adapted to salt-water surroundings. They are mainly saxicolous, but the presence of sea-water is the factor of greatest influence on their growth and distribution, and they occur indifferently on any kind of shore rock either siliceous or calcareous. Wheldon and Wilson[1185] noted this indifference to substratum on the Arran shores, where a few calcicolous species such as Verrucaria nigrescens, V. maculiformis, Placodium tegularis and Pl. lobulatum, grow by the sea on siliceous rocks. They suggest that the spray-washed habitat affords the conditions, which, in other places, are furnished by limestone.

The greater or less proximity of the salt water induces in lichens, as in other maritime plants, a distribution into belts or zones which recede gradually or abruptly according to the slope of the shore and the reach of the tide. Weddell[1186] on the Isle d’Yeu delimited three such zones: (1) marine, those nearest the sea and immersed for a longer or shorter period at each tide; (2) semi-marine, not immersed but subject to the direct action of the waves, and (3) maritime or littoral, the area beyond the reach of the waves but within the influence of sea-spray. In the course of his work he indicates the lichens of each zone.

Fig. 122. Ramalina siliquosa A. L. Sm. Upper zone of barren plants (after M. C. Knowles, R. Welch. Photo.).

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. 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:

Fig. 123. Ramalina siliquosa A. L. Sm. Lower zone of fertile plants (after M. C. Knowles, R. Welch, Photo.).

Fig. 124. Sward of young Ramalinae (after M. C. Knowles, R. Welch, Photo.).

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.

Fig. 125. Crustaceous communities in the Ramalina belt. Lecanora atra Ach. (grey patches) and Buellia ryssolea A. L. Sm. (dark patches). (After M. C. Knowles, R. Welch, Photo.)

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:

“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:

(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:

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:

While in a higher position on similar boulders:

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:

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.

FormationAssociationPrincipal Species
Dune1. Bare SandCetraria aculeata f. acanthella
Cladonia furcata
2. Grey DuneCladonia rangiferina, Peltigera rufescens
Cladonia furcata, Cl. alcicornis
3. Derelict DuneCladonia furcata, Parmelia fuliginosa
Rhizocarpon confervoides
Shingle4. High ShingleLooseWith sandLecanora atra, L. galactina
Rhizocarpon confervoides
Lecanora citrina
Without sandPhyscia tenella, Lecanora citrina, Xanthoria parietina
Squamaria saxicola
Parmelia saxatilis, P. fuliginosa
BoundWith grassesCladonia rangiferina, Cl. furcata, Cl. pungens
Cetraria aculeata
Without grassesXanthoria parietina, Biatorina chalybeia, Lecanora atra
Aspicilia gibbosa, Buellia colludens, Verrucaria microspora
Physcia tenella, Lecanora atroflava
5. Low ShingleRhizocarpon confervoides, Lecanora citrina var. incrustans
L. badia, L. atra, Xanthoria parietina
Verrucaria maura

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:

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:

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:

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:

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.