Chapter IX: Ecology (1)

A. GENERAL INTRODUCTION

Ecology is the science that deals with the habitats of plants and their response to the environment of climate or of substratum. Ecology in the lichen kingdom is habitat “writ large,” and though it will not be possible in so wide a field to enter into much detail, even a short examination of lichens in this aspect should yield interesting results, especially as lichens have never, at any time, been described without reference to their habitat. In very early days, medicinal Usneas were supposed to possess peculiar virtues according to the trees on which they grew and which are therefore carefully recorded, and all down the pages of lichen literature, no diagnosis has been drawn up without definite reference to the nature of the substratum. Not only rocks and trees are recorded, but the kind of rock and the kind of tree are often specified. The important part played by rock lichens in preparing soil for other plants has also received much attention[1116].

Several comprehensive works on Ecology have been published in recent times and though they deal mainly with the higher vegetation, the general plan of study of land plants is well adapted to lichens. A series of definitions and explanations of the terms used will be of service:

Thus in a work by Moss[1117] we read “The flora is composed of the individual species: the vegetation comprises the groupings of these species into ensembles termed vegetation units or plant communities.” And again:

1. “A _plant formation_ is the whole of the vegetation which occurs on a definite and essentially uniform habitat.”—All kinds of plants are included in the formation, so that strictly speaking a _lichen formation_ is one in which lichens are the dominant plants. Cf. p. 394. The term however is very loosely used in the literature. A uniform habitat, as regards lichens, would be that of the different kinds of soil, of rock, of tree, etc.

2. “A _plant association_ is of lower rank than a formation, and is characterized by minor differences within the generally uniform habitat.”—It represents a more limited community within the formation.

3. “A _plant society_ is of lower rank than an association, and is marked by still less fundamental differences of the habitat.”—The last-named term represents chiefly aggregations of single species. Moss adds that: “_plant community_ is a convenient and general term used for a vegetation unit of any rank.”

Climatic conditions and geographical position are included in any consideration of habitat, as lichens like other plants are susceptible to external influences.

Ecological plant-geography has been well defined by Macmillan[1118] as “the science which treats of the reciprocal relation between physiographic conditions and life requirements of organisms in so far as such relations manifest themselves in choice of habitats and method of establishment upon them ... resulting in the origin and development of plant formations.”

B. EXTERNAL INFLUENCES

The climatic factors most favourable to lichen development are direct light (already discussed)[1119], a moderate or cold temperature, constant moisture and a clear pure atmosphere. Wind also affects their growth.

_a._ _TEMPERATURE._ Lichens, as we have seen, can endure the heat of direct sunlight owing to the protection afforded by thickened cortices, colour pigments, etc. Where such heat is so intense as to be injurious the gonidia succumb first[1120].

Lichens endure low temperatures better than other plants, their xerophytic structure rendering them proof against extreme conditions: the hyphae have thick walls with reduced cell lumen and extremely meagre contents. Freezing for prolonged periods does them little injury; they revive again when conditions become more favourable. Efficient protection is also afforded by the thickened cortex of such lichens as exist in Polar areas, or at high altitudes. Thus various species of _Cetrariae_ with a stout “decomposed” amorphous cortex can withstand very low temperatures and grow freely on the tundra, while _Cladonia rangiferina_, also a northern lichen, but without a continuous cortex, cannot exist in such cold conditions, unless in localities where it is protected by a covering of snow during the most inclement seasons.

_b._ _HUMIDITY._ A high degree of humidity is distinctly of advantage to the growth of the lichen thallus, though when the moist conditions are excessive the plants become turgid and soredial states are developed.

The great abundance of lichens in the western districts of the British Isles, where the rainfall is heaviest, is proof enough of the advantage of moisture, and on trees it is the side exposed to wind and rain that is most plentifully covered. A series of observations on lichens and rainfall were made by West[1121] and have been published since his death. He has remarked in more than one of his papers that a most favourable situation for lichen growth is one that is subject to a drive of wind with much rain. In localities with an average of 216 days of rain in the year, he found abundant and luxuriant growths of the larger foliose species. In West Ireland there were specimens of _Ricasolia laetevirens_ measuring 165 by 60 cm. In West Scotland with an “average of total days of rain, 225,” he found plants of _Ricasolia amplissima_ 150 × 90 cm. in size, of _R. laetevirens_ 120 × 90 cm., while _Pertusaria globulifera_ formed a continuous crust on the trees as much as 120 × 90 cm. _Lecanora tartarea_ seemed to thrive exceptionally well when subject to driving mists and rains from mountain or moorland, and was in these circumstances frequently the dominant epiphyte. Bruce Fink[1122] also observed in his ecological excursions that the number of species and individuals was greater near lakes or rivers.

Though a fair number of lichens are adapted to life wholly or partly under water, land forms are mostly xerophytic in structure, and die off if submerged for any length of time. The _Peltigerae_ are perhaps the most hydrophilous of purely land species. Many Alpine or Polar forms are covered with snow for long periods. In the extreme north it affords more or less protection; and Kihlman[1123] and others have remarked on the scarcity of lichens in localities denuded of the snow mantle and exposed to severe winter cold. On the other hand lichens on the high Alpine summits that are covered with snow the greater part of the year suffer, according to Nilson[1124], from the excessive moisture and the deprivation of light. Foliose and fruticose forms were, he found, dwarfed in size; the crustaceous species had a very thin thallus and in all of them the colour was impure. _Gyrophorae_ seemed to be most affected: folds and outgrowths of the thallus were formed and the internal tissues were partly disintegrated. Lichens on the blocks of the glacier moraines which are subject to inundations of ice-cold water after the snow has melted, were unhealthy looking, poorly developed and often sterile, though able to persist in a barren state. Lindsay[1125] noted as a result of such conditions on _Cladoniae_ not only sterility but also deformity both of vegetative and reproductive organs; discolouration and mottling of the thallus and an increased development of squamules of the primary thallus and on the podetia.

_c._ _WIND._ Horizontal crustaceous or foliose lichens are not liable to direct injury by wind as their close adherence to the substratum sufficiently shelters them. It is only when the wind carries with it any considerable quantity of sand that the tree or rock surfaces are swept bare and prevented from ever harbouring any vegetation, and also, as has been already noted, the terrible winds round the poles are fatal to lichens exposed to the blasts unless they are provided with a special protective cortex. After crustaceous forms, species of _Cetraria_, _Stereocaulon_ and _Cladonia_ are best fitted for weathering wind storms: the tufted[1126] cushion-like growth adopted by these lichens gives them mutual protection, not only against wind, but against superincumbent masses of snow. Kihlman[1123] has given us a vivid account of wind action in the Tundra region. He noted numerous hollows completely scooped out down to the sand: in these sheltered nooks he observed the gradual colonization of the depressions, first by a growth of hepatics and mosses and by such ground lichens as _Peltigera canina_, _P. aphthosa_ and _Nephromium arcticum_; they cover the soil and in time the hollow becomes filled with a mass of vegetation consisting of Cladonias, mosses, etc. On reaching a certain more exposed level these begin to wither and die off at the tips, killed by the high cold winds. Then arrives _Lecanora tartarea_, one of the commonest Arctic lichens, and one which is readily a saprophyte on decayed vegetation. It covers completely the mound of weakened plants which are thus smothered and finally killed. The collapse of the substratum entails in turn the breaking of the _Lecanora_ crust, and the next high wind sweeps away the whole crumbling mass. How long recolonization takes, it was impossible to find out.

Upright fruticose lichens are necessarily more liable to damage by wind, but maritime _Ramalinae_ and _Roccellae_ do not seem to suffer in temperate climates, though in regions of extreme cold fruticose forms are dwarfed and stunted. The highest development of filamentous lichens is to be found in more or less sheltered woods, but the effect of wind on these lichens is not wholly unfavourable. Observations have been made by Peirce[1127] on two American pendulous lichens which are dependent on wind for dissemination. On the Californian coasts a very large and very frequent species, _Ramalina reticulata_ (Fig. 64), is seldom found undamaged by wind. In Northern California the deciduous oaks _Quercus alba_ and _Q. Douglasii_ are festooned with the lichen, while the evergreen “live oak,” _Q. chrysolepis_, with persistent foliage, only bears scraps that have been blown on to it. Nearer the coast and southward the lichen grows on all kinds of trees and shrubs. The fronds of this _Ramalina_ form a delicate reticulation and when moist are easily torn. In the winter season, when the leaves are off the trees, wind- and rain-storms are frequent; the lichen is then exposed to the full force of the elements and fragments and shreds are blown to other trees, becoming coiled and entangled round the naked branches and barky excrescences, on which they continue to grow and fruit perfectly well. A succeeding storm may loosen them and carry them still further. Peirce noted that only plants developed from the spore formed hold-fasts and they were always small, the largest formed measuring seven inches in length. Both the hold-fast and the primary stalk were too slight to resist the tearing action of the wind.

Schrenk[1128] made a series of observations and experiments with the lichens _Usnea plicata_ and _U. dasypoga_, long hanging forms common on short-leaved conifers such as spruce and juniper. The branches of these trees are often covered with tangled masses of the lichens not due to local growth, but to wind-borne strands and to coiling and intertwining of the filaments owing to successive wetting and drying. Tests were made as to the force of wind required to tear the lichens and it was found that velocities of 77 miles per hour were not sufficient to cause any pieces of the lichen to fly off when it was dry; but after soaking in water, the first pieces were torn off at 50 miles an hour. These figures are, however, considered by Schrenk to be too high as it was found impossible in artificially created wind to keep up the condition of saturation. It is the combination of wind and rain that is so effective in ensuring the dispersal of both these lichens.

_d._ _HUMAN AGENCY._ Though lichens are generally associated with undisturbed areas and undisturbed conditions, yet accidents or convulsions of nature, as well as changes effected by man, may at times prove favourable to their development. The opening up of forests by thinning or clearing will be followed in time by a growth of tree and ground forms; newly planted trees may furnish a new lichen flora, and the building of houses and walls with their intermixture of calcareous mortar will attract a particular series of siliceous or of lime-loving lichens. A few lichens are partial to the trees of cultivated areas, such as park-lands, avenues or road-sides. Among these are several species of _Physcia_: _Ph. pulverulenta_, _Ph. ciliaris_ and _Ph. stellaris_, some species of _Placodium_, and those lichens such as Lecanora varia that frequently grow on old palings.

On the other hand lichens are driven away from areas of dense population, or from regions affected by the contaminated air of industrial centres. In our older British Floras there are records of lichens collected in London during the eighteenth century—in Hyde Park and on Hampstead Heath—but these have long disappeared. A variety of _Lecanora galactina_ seems to be the only lichen left within the London district: it has been found at Camden Town, Notting Hill and South Kensington.

So recently as 1866, Nylander[1129] made a list of the lichens growing in the Luxembourg gardens in Paris; the chestnuts in the alley of the Observatory were the most thickly covered, and the list includes about 35 different species or varieties, some of them poorly developed and occurring but rarely, others always sterile, but quite a number in healthy fruiting condition. All of them were crustaceous or squamulose forms except _Parmelia acetabulum_, which was very rare and sterile; _Physcia obscura_ var. and _Ph. pulverulenta_ var., also sterile; _Physcia stellaris_ with occasional abortive apothecia and _Xanthoria parietina_, abundant and fertile. In 1898, Hue[1130] tells us, there were no lichens to be found on the trees and only traces of lichen growth on the stone balustrades.

The question of atmospheric pollution in manufacturing districts and its effect on vegetation, more especially on lichen vegetation, has received special attention from Wheldon and Wilson[1131] in their account of the lichens of South Lancashire, a district peculiarly suitable for such an inquiry, as nowhere, according to the observations, are the evil effects of impure air so evident or so wide-spread. The unfavourable conditions have prevailed for a long time and the lichens have consequently become very rare, those that still survive leading but a meagre existence. The chief impurity is coal smoke which is produced not only from factories but from private dwellings, and its harmful effect goes far beyond the limits of the towns or suburbs, lichens being seen to deteriorate as soon as there is the slightest deposition of coal combustion products—especially sulphur compounds—either on the plants or on the surfaces on which they grow. The larger foliose and fruticose forms have evidently been the most severely affected. “While genera of bark-loving lichens such as _Calicium_, _Usnea_, _Ramalina_, _Graphis_, _Opegrapha_, _Arthonia_ etc. are either wholly absent or are poorly represented in the district,” corticolous species now represent about 15 per cent. of those that are left; those that seem best to resist the pernicious influences of the smoky atmosphere are, principally, _Lecanora varia, Parmelia saxatilis, P. physodes_ and to a less degree _P. sulcata_, _P. fuliginosa_ var. _laetevirens_ and _Pertusaria amara_.

Saxicolous lichens have also suffered severely in South Lancashire; not only the number of species, but the number of individuals is enormously reduced and the specimens that have persisted are usually poorly developed. The smoke-producing towns are situated in the valley-bottoms, and the smoke rises and drifts on to the surrounding hills and moorlands. The authors noted that crustaceous rock-lichens were in better condition on horizontal surfaces such as the copings of walls, or half-buried stones, etc. than on the perpendicular or sloping faces of rocks or walls. This was probably due to what they observed as to the effect of water trickling down the inclined substrata and becoming charged with acid from the rock surfaces. They also observed further that a calcareous substratum seemed to counteract the effect of the smoke, the sulphuric acid combining with the lime to form calcium sulphate, and the surface-washings thus being neutralized, the lichens there are more favourably situated. They found in good fruiting condition, on mortar, cement or concrete, the species _Lecanora urbana_, _L. campestris_, _L. crenulata_, _Verrucaria muralis_, _V. rupestris_, _Thelidium microcarpum_ and _Staurothele hymenogonia_. Some of these occurred on the mortar of sandstone walls close to the town, “whilst on the surface of the sandstone itself no lichens were present.”

Soil-lichens were also strongly affected, the Cladoniae of the moorlands being in a very depauperate condition, and there was no trace of _Stereocaulon_ or of _Sphaerophorus_ species, which, according to older records, previously occurred on the high uplands.

The influence of human agency is well exemplified in one of the London districts. In 1883 Crombie published a list of the lichens recorded from Epping Forest during the nineteenth century. They numbered 171 species, varieties or forms, but, at the date of publication, many had died out owing to the destruction of the older trees; the undue crowding of the trees that were left and the ever increasing population on the outskirts of the Forest. Crombie himself made a systematic search for those that remained, and could only find some 85 different kinds, many of them in a fragmentary or sterile condition.

R. Paulson and P. Thompson[1132] commenced a lichen exploration of the Forest 27 years after Crombie’s report was published, and they have found that though the houses and the population have continued to increase round the area, the lichens have not suffered. “Species considered by Crombie as rare or sterile are now fairly abundant, and produce numerous apothecia. Such are _Baeomyces rufus_, _B. roseus_, _Cladonia pyxidata_, _Cl. macilenta_ var. _coronata_, _Cl. Floerkeana_ f. _trachypoda_, _Lecanora varia_, _Lecidea decolorans_ and _Lecidea tricolor_.” They conclude that “some at least of the Forest lichens are in a far more healthy and fertile condition than they were 27 years ago.” They attribute the improvement mainly to the thinning of trees and the opening up of glades through the Forest, letting in light and air not only to the tree trunks but to the soil. In 1912[1133] the authors in a second paper reported that 109 different kinds had been determined, and these, though still falling far short of the older lichen flora, considerably exceed the list of 85 recorded in 1883.

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:

A species of _Thelotrema_ on _Cinchona oblongifolia_.
A species of _Chiodecton_ on _C. cordifolia_.
A species of _Sarcographa_ on _C. condaminea_.

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:

_Xanthoria polycarpa._
_Candelaria concolor._
_Parmelia olivacea_, _P. adglutinata_.
_Placodium cerinum._
_Lecanora subfusca._
_Bacidia fusca-rubella._
_Lecidea enteroleuca._
_Graphis scripta._
_Arthonia lecideella_, _A. dispersa_.
_Arthopyrenia punctiformis_, _A. fallax_.
_Pyrenula nitida_, _P. thelena_, _P. cinerella_, _P. leucoplaca_.

On rough bark he records:

_Ramalina calicaris_, _R. fraxinea_, _R. fastigiata_.
_Teloschistes chrysophthalmus._
_Xanthoria polycarpa_, _X. lychnea_.
_Candelaria concolor._
_Parmelia perforata_, _P. crinita_, _P. Borreri_, _P. tiliacea_,
_P. saxatilis_, _P. caperata_.
_Physcia granulifera_, _Ph. pulverulenta_, _Ph. stellaris_, _Ph.
tribacia_, _Ph. obscura_.
_Collema pycnocarpum_, _C. flaccidum_.
_Leptogium mycochroum._
_Placodium aurantiacum_, _Pl. cerinum_.
_Lecanora subfusca._
_Pertusaria leioplaca_, _P. velata_.
_Bacidia rubella_, _B. fuscorubella_.
_Lecidea enteroleuca._
_Rhizocarpon alboatrum_, _Buellia parasema_.
_Opegrapha varia._
_Graphis scripta._
_Arthonia lecideella_, _A. radiata_.
_Arthopyrenia quinqueseptata_, _A. macrospora_.
_Pyrenula nitida_, _P. leucoplaca_.

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:

_Collema flaccidum._
_Calicium hyperellum._
_Ramalina calicaris_, _R. fraxinea_ with var. _ampliata_, _R.
fastigiata_, _R. farinacea_ and _R. pollinaria_.
_Parmelia saxatilis_ and f. _furfuracea_, _P. caperata_, _P. physodes_.
_Physcia pulverulenta_, _Ph. tenella_ (_hispida_).
_Lecanora subfusca_, _L. rugosa_.
_Pertusaria amara_, _P. globulifera_, _P. communis_, _P. Wulfenii_.
_Lecidea_ (_Buellia_) _canescens_.
_Graphis scripta._

And on the soil of these woods:

_Cladonia pyxidata_, _Cl. pungens_, _Cl. macilenta_, _Cl. pityrea_,
_Cl. squamosa_ and _Cl. sylvatica_.

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:

_Usnea hirta._
_Cetraria diffusa._
_Evernia furfuracea._
_Parmelia scortia_, _P. physodes_.
_Xanthoria parietina._
_Placodium cerinum._
_Rinodina exigua._
_Lecanora Hageni_, _L. varia_ and its allies.
_Lecidea ostreata_, _L. parasema_.
_Buellia myriocarpa._
Cladoniaceae and Caliciaceae (several species).

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:

_Teloschistes chrysophthalmus._
_Placodium cerinum._
_Lecanora Hageni_, _L. varia_.
_Rinodina sophodes_, _R. exigua_.
_Buellia parasema_ (_disciformis_), _B. turgescens_.
_Calicium parietinum._
_Thelocarpon prasinellum._

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:

_Heppia Despreauxii._
_Urceolaria scruposa._
_Biatora_ (_Lecidea_) _decipiens_.
_Biatora_ (_Bacidia_) _muscorum_.
_Dermatocarpon hepaticum._

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:

_Cladonia sylvatica_ f. _alpestris_.
_Cladonia squamosa_ f. _subsquamosa_.
_Cladonia rangiformis_ f. _foliosa_.
_Cladonia cariosa_ and f. _symphicarpa_.
_Peltigera canina_ f. _soreumatica_.
_Solorina spongiosa._
_Heppia virescens._
_Lecanora crassa._
_Urceolaria scruposa_ f. _argillacea_.
_Verrucaria_ (_Thrombium_) _epigaea_.
_Lecidea decipiens._
_Dermatocarpon cinereum._
_Collema granulatum._
_Collema tenax._
_Leptogium byssinum._

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:

_Baeomyces roseus._
_Baeomyces rufus._
_Baeomyces placophyllus._
_Endocarpon_ spp.
_Gongylia viridis._
_Dermatocarpon lachneum_
_Dermatocarpon hepaticum._
_Dermatocarpon cinereum._

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:

_Cladonia coccifera._
_Cladonia agariciformis._
_Secoliga_ (_Gyalecta_) _bryophaga_.
_Baeomyces rufus._
_Lecidea gelatinosa._
_Psorotichia lutophila._

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:

_Baeomyces roseus._
_Baeomyces rufus._
_Pycnothelia papillaria._
_Cladonia coccifera._
_Lecidea granulosa._
_Cladonia macilenta._
_Cladonia furcata._
_Cetraria aculeata._
_Peltigera spuria._

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.

_Verrucaria nigrescens._
_Lecidea coarctata._
_Candelariella vitellina._
_Lecanora dispersa._
_Lecanora galactina._
_Lecanora Hageni._
_Lecanora saxicola._
_Parmelia conspersa._
_Placodium teicholytum._
_Placodium pyraceum._
_Placodium decipiens._
_Placodium elegans._
_Placodium murorum._
_Xanthoria parietina._
_Rhizocarpon alboatrum_ var.
_Buellia myriocarpa._
_Lecidea demissa._
_Physcia ascendens._
_Physcia caesia._
_Physcia obscura._
_Physcia sciastrella._

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:

_Cladonia furcata._
_Cladonia cristatella._
_Cladonia gracilis._
_Cladonia verticillata._
_Cladonia rangiferina._
_Cladonia uncialis._
_Cladonia alpestris._
_Cladonia turgida._
_Cladonia coccifera._
_Cladonia pyxidata._
_Cladonia fimbriata._
_Peltigera malacea._
_Peltigera canina._
_Peltigera aphthosa._

_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:

_Cladonia Floerkeana._
_Biatora terricola._
_Thelocarpon turficolum._
_Geisleria sychnogonioides._

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:

_Leptogrum_ spp.
_Peltigera_ spp.
_Cetraria_ spp.
_Parmelia physodes._
_Psoroma hypnorum._
_Lecanora epibryon._
_Lecanora tartarea._
_Lecidea coarctata._
_Lecidea granulosa._
_Lecidea uliginosa._
_Lecidea neglecta._
_Bilimbia sabulosa._
_Bilimbia liguiaria._
_Bilimbia melaena._
_Baeomyces_ spp.
_Cladonia_ spp.

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:

_Pannaria_ (_Placynthium_) _nigra_.
_Crocynia lanuginosa._
_Omphalaria pulvinata._
_Collema plicatile._
_Collema pustulatum._
_Leptogium lacerum._
_Placodium citrinum._
_Bacidia inundata._
_Rhizocarpon alboatrum_ var.
_Dermatocarpon miniatum._
_Staurothele umbrinum._

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:

_Acarospora chlorophana._
_Lecanora glaucoma._
_Rhizocarpon viridiatrum._
_Lecidea tumida._
_Biatorella sporostatia._
_Biatorella testudinea._

On granite and quartzite disintegrated:

_Aspicilia cinerea._
_Aspicilia gibbosa._
_Aspicilia tenebrosa._
_Buellia coracina._
_Catillaria_ (_Biatorina_) _Hochstetteri_.
_Rhizocarpon petraeum._
_Rhizocarpon geographicum_ vars.
_Biatorella cinerea._
_Lecanora badia._
_Lecanora cenisia._
_Lecidea confluens._
_Lecidea fuscoatra._
_Lecidea platycarpa._
_Lecidea lapicida._
_Haematomma ventosum._

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:

_Aspicilia cinerea._
_Haematomma ventosum._
_Rhizocarpon geographicum_ vars.
_Biatorella cinerea._
_Lecidea platycarpa._

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:

_Sphaerophorus coralloides._
_Sphaerophorus fragilis._
_Platysma Fahlunense._
_Platysma commixtum._
_Platysma glaucum._
_Platysma lacunosum._
_Parmelia saxatilis._
_Parmelia omphalodes._
_Parmelia Mougeotii._
_Parmelia stygia._
_Parmelia tristis._
_Parmelia lanata._
_Gyrophora proboscidea._
_Gyrophora cylindrica._
_Gyrophora torrefacta._
_Gyrophora polyphylla._
_Gyrophora flocculosa._
_Lecanora gelida._
_Lecanora atra._
_Lecanora badia._
_Lecanora tartarea._
_Lecanora parella._
_Lecanora ventosa._
_Lecanora Dicksonii._
_Lecanora cinerea._
_Lecanora peliocypha._
_Pertusaria dealbata._
_Stereocaulon Delisei._
_Stereocaulon evolutum._
_Stereocaulon coralloides._
_Stereocaulon denudatum._
_Psorotichia lugubris._
_Lecidea inserena._
_Lecidea panaeola._
_Lecidea contigua._
_Lecidea confluens._
_Lecidea lapicida._
_Lecidea plana._
_Lecidea mesotropa._
_Lecidea auriculata._
_Lecidea diducens._
_Lecidea aglaea._
_Lecidea rivulosa._
_Lecidea Kochiana._
_Lecidea pycnocarpa._
_Buellia atrata._
_Rhizocarpon Oederi._

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_.