Chapter IX: Ecology (2)

=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.

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

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

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

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

In the _Ramalina_ zone along with the dominant species there occur occasional tufts of _R. Curnowii_ and _R. subfarinacea_, the latter more especially in shady and rather moist situations. There are also numerous foliaceous and crustaceous lichens mingling with the _Ramalina_ vegetation (Fig. 125), several Parmelias, _Physcia aquila_, _Xanthoria parietina_, _Buellia canescens_, _B. ryssolea_, _Lecanora atra_, _L. sordida_, _Rhizocarpon geographicum_ and others. In the main these are arranged in the following order descending towards the sea:

1. _Parmeliae._
2. _Physcia aquila._
3. _Xanthoria parietina._
4. Crustaceous species.

_Parmelia prolixa_ is the most abundant of the Parmelias: it covers large spaces of the rocks and frequently competes for room with the Ramalinas, or in other areas with _Physcia aquila_ and _Lecanora parella_.

A number of crustaceous species which form the sub-vegetation of the _Ramalina_ belt, and also on the same level, clothe the steeper rock faces where shelter and moisture are insufficient to support the foliose forms. “In general the sub-vegetation of the eastern and northern coasts is largely composed of species that are common in Alpine and upland regions. This is due to the steepness of the rocks and also to the colder and drier conditions prevailing on these coasts.” An association of _Rhizocarpon geographicum_, _Lecanora_ (_sordida_) _glaucoma_ and _Pertusaria concreta_ f. _Westringii_ forms an almost continuous covering in some places, descending nearly to sea-level.

On sunnier and moister rocks with a south and south-west aspect the association is of more lowland forms such as _Buellia colludens_, _B. stellulata_, _Lecanora smaragdula_ and _L. simplex_ f. _strepsodina_.

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

1. _Xanthoria parietina._
2. _Placodium murorum._
3. _Placodium tegularis._
4. _Placodium decipiens._
5. _Placodium lobulatum._

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

_Lecanora prosechoides._
_Lecanora umbrina._
_Lecanora Hageni._
_Rhizocarpon alboatrum._
_Biatorina lenticularis._
_Rinodina exigua var. demissa._
_Opegrapha calcarea f. heteromorpha._

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

_Verrucaria maura_ is one of the most abundant lichens of our rocky coasts, and is reported from Spitzbergen in the North to Graham Land in the Antarctic. It grows well within the range of sea-spray, covering great stretches of boulders and rocks with its dull-black crustaceous thallus. At Howth it is submerged only by the highest spring tides. Though it is the dominant lichen on that beach, other species such as _V. memnonia_, _V. prominula_, and _V. aquatilis_ form part of the association, and more rarely _V. scotina_ along with _Arthopyrenia halodytes_, _A. leptotera_ and _A. halizoa_.

(5) =The belt of marine Verrucarias.= This association includes the species that are submerged by the tide for a longer or shorter period each day. The dominant species are _Verrucaria microspora_, _V. striatula_ and _V. mucosa_. _Arthopyrenia halodytes_ is also abundant; _A. halizoa_ and _A. marina_ are more rarely represented. Among the plants of _Fucus spiralis_, _Verrucaria mucosa_, the most wide-spreading of these marine forms, is “very conspicuous as a dark-green, almost black, band of greasy appearance stretching along the shore.” When growing in the shade, the thallus is of a brighter green colour.

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

_Physcia aquila._
_Xanthoria parietina._
_Lecanora parella._
_Lecanora atra._
_Lecanora campestris._
_Placodium ferrugineum_ var. _festivum._
_Placodium tegularis._
_Ramalina cuspidata._
_Physcia stellaris._
_Physcia tenella._
_Verrucaria maura._

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

_Verrucaria maura._
_Lichina confinis._
_Lecanora prosechoides._
_Placodium lobulatum._

While in a higher position on similar boulders:

_Lecanora exigua._
_Lecanora dispersa._
_Lecanora galactina._
_Lecanora sulphurea._
_Lecanora saxicola._
_Lecanora caesiocinerea._
_Lecanora gibbosa._
_Lecanora atra._
_Lecanora parella._
_Lecidea colludens._
_Lecidea lavata._
_Lecidea nigroclavata_ f. _lenticularis._
_Xanthoria parietina_ and f. _aureola._
_Physcia subobscura._
_Physcia caesia._

And more rarely a few species of _Lecidea_.

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

_Parmelia physodes._
_Evernia prunastri._
_Cetraria aculeata._
_Cladonia cervicornis._
_Cladonia endiviaefolia._
_Peltigera_ spp.

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

When the loose sand on the dunes of South Lancashire becomes cemented by algae and mosses several rare _Lecideae_ are to be found on the decaying vegetation, and with further accumulation of humus _Cladoniae_ appear and spread rapidly along with several species of _Peltigera_ and the ubiquitous _Parmelia physodes_. The latter starts on dead twigs of _Salix repens_ and spreads on to the surrounding soil where it forms patches some inches in diameter. The association also includes _Lecidea uliginosa_ and _Bilimbia sphaeroides_.

On the more inland portions of the dunes numerous rather poorly developed _Cladoniae_ and _Cetraria aculeata_ were associated, while on the sides of “slacks” or “dune-pans” _Collema pulposum_, _Cladonia sylvatica_ and several crustaceous lichens covered the soil. The wetter parts of the dunes were not found to be favourable to lichen growth.

Sandstede[1196] found on the sandy shores of Rügen, from the shore upwards: first a stretch of bare sand, then a few dune grasses with scattered scraps of _Cladoniae_, _Peltigerae_ and _Cetraria aculeata_. Next in order sandbanks with _Parmelia physodes_, _Cladonia sylvatica_, _Cl. alcicornis_ and _Stereocaulon paschale_. All these are species that occur on similar shores in the British Islands. Sandstede adds an extensive list of maritime species observed by him in Rügen.

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

(1) =Bare sand=, which is the first association listed, is an area practically without phanerogams; the few lichen plants, _Cladonia furcata_ and _Cetraria aculeata_ f. _acanthella_, are attached by slight embedding in the soil.

(2) =Grey dune.= The sand-loving lichens of the association grow in company with _Hypnum cupressiforme_ and attain their greatest development. Other species which also occur there are _Parmelia physodes_ and _Evernia prunastri_ var. _stictocera_.

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

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

(5) =Low shingle.= This last association lies in the hollows among plants of _Suacda fruticosa_. Stability is high and tidal immersions regular and frequent. The dominant factor of the association is the quantity of humus and mud deposited around and over the stones. The lichens cover almost every available spot on the firmly embedded pebbles. The characteristic species of such areas are _Lecanora badia_ and _L._ (_Placodium_) _citrina_ which effect the primary colonization. To these succeed _Lecanora atra_ and _Xanthoria parietina_. In time the mud overwhelms and partly destroys the lichens, so that the phase of luxuriant growth is only temporary.

_Lecanora badia_ is conspicuously abundant at the sand end of this formation. _Lecanora_ (_Placodium_) _citrina_ disappears as the mud is left behind. _Collema_ spp. also occur frequently on the mixture of mud and sand round the stones. The species on “low shingle” are those most tolerant of submersion: _Verrucaria maura_ is confined to this area, where it is covered by the tide several hours each day.

FORMATION ASSOCIATION PRINCIPAL SPECIES

{1. Bare Sand _Cetraria aculeata_ f. _acanthella_
{ _Cladonia furcata_
Dune {2. Grey Dune _Cladonia rangiferina_,
{ _Peltigera rufescens_
{ _Cladonia furcata_, _Cl. alcicornis_
{3. Derelict Dune _Cladonia furcata_, _Parmelia fuliginosa_
{ _Rhizocarpon confervoides_

{4. High Shingle
{ {_Lecanora atra_, _L. galactina_
{ {With sand {_Rhizocarpon confervoides_
{ { {_Lecanora citrina_
{ Loose
{ { {_Physcia tenella_, _Lecanora citrina_,
{ {Without sand {_Xanthoria parietina_
{ { {_Squamaria saxicola_
{ {_Parmelia saxatilis_, _P. fuliginosa_
Shingle{
{ {_Cladonia rangiferina_, _Cl. furcata_,
{ {With grasses {_Cl. pungens_
{ { {_Cetraria aculeata_
{ Bound
{ { {_Xanthoria parietina_,
{ { {_Biatorina chalybeia_,
{ {Without {_Lecanora atra_
{ { grasses {_Aspicilia gibbosa_, _Buellia colludens_,
{ { {_Verrucaria microspora_
{ {_Physcia tenella_, _Lecanora atroflava_
{
{ _Rhizocarpon confervoides_,
{ _Lecanora citrina_ var. _incrustans_
{5. Low Shingle _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:

_Euopsis granatina._
_Sphaerophorus coralloides._
_Sphaerophorus fragilis._
_Gyrophora polyphylla._
_Cetraria tristis._
_Cetraria nivalis._
_Lecanora tartarea_ var. _frigida_.
_Lecanora upsaliensis._
_Aspicilia oculata._
_Pertusaria dactylina._
_Pertusaria glomerata._
_Stereocaulon denudatum._
_Parmelia saxatilis._
_Parmelia omphalodes._
_Parmelia lanata._
_Parmelia stygia._
_Stereocaulon tomentosum._
_Stereocaulon alpinum._
_Cladonia coccinca._
_Cladonia gracilis._
_Cladonia uncialis._
_Cladonia destricta._
_Cladonia racemosa._
_Lecidea arctica._
_Parmelia alpicola._
_Cetraria aculeata._
_Cetraria crispa._
_Cetraria islandica._
_Lecidea limosa._
_Lecidea alpestris._
_Lecidea demissa._
_Lecidea uliginosa._
_Lecidea cuprea._
_Lecidea Berengeriana._
_Lecidea cupreiformis._
_Lecidea atrofusca._

Again on the summit of Ben-y-Gloe the same authors[1199] have recorded _Gyrophora erosa_, _G. torrefacta_ and _G. cylindrica_, _Parmelia alpicola_, _Lecanora tartarea_ var. _frigida_, _Lecidea limosa_ and _L. arctica_, the last two lichens thriving in the most bleak and exposed situations. _Cladonia cervicornis_ grew in reduced squamulose cushions; _Stereocaulon_ and _Sphaerophorus_ in very compact forms, the outer stalks prostrate, the next inclined, the central ones erect so that points only are exposed and no lateral stress is caused by wind storms. Erect fruticose lichens are absent in this region, being represented only by _Parmelia lanata_, a semi-decumbent plant, and by _Thamnolia vermicularis_ which is prostrate on the ground except where the points of the stalks turn up to catch the dew. Many of the _Lecideae_ were observed to have large fruits and very little thallus: “the hyphae ramify in the minute interstices of the stone and the gonidia cluster under the lea of the apothecia: this is especially the case on loose stones where conditions are extremely dry.”

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

_Stereocaulon condensatum._
_Gyrophora cylindrica._
_Gyrophora spodochroa._
_Solorina crocea._
_Solorina saccata._
_Parmelia encausta._
_Candelaria concolor._
_Caloplaca pyracea_ var. _nivalis_.
_Haematomma ventosum._
_Acarospora smaragdula._
_Psora decipiens._
_Buellia discolor._
_Buellia stellulata._
_Lecidea contigua_ var. _steriza_.
_Lecidea confluens._
_Dermatocarpon hepaticum._

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

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

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

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

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

The same kinds of lichens occur all over these northern regions. Birger Nilson[1203] gives as the principal earth-lichens in Swedish Lappland, _Alectoria ochroleuca_, _A. nigricans_, _Cetraria nivalis_, _C. cucullata_, _Cladonia uncialis_, _Thamnolia_ (_Cerania_) _vermicularis_ and _Sphaerophorus coralloides_.

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

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

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

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

At 5000 feet and upwards lichens are, in general, exceedingly abundant on all rock substrata and represent 57 species or subspecies, only three of these being arboreal: _Buellia triphragmia_ occurs rarely, _Xanthoria polycarpa_ is frequent on sage brush, while _Candelariella cerinella_ though a rock-lichen grows occasionally on the same substratum. _Caloplaca_ (_Placodium_) _elegans_ is one of the most successful and abundant species and along with _Lecanora_ (nine forms), _Acarospora_ (seven forms) and _Lecidia_ (five forms) comprises three-fourths of the rock surface occupied by lichens. The addition of _Rinodina_ with two species and _Gyrophora_ with four brings the computation of individuals in these desert rock formations up to nine-tenths of the whole. As the desert rocks pass to the Alpine, _Gyrophora_ becomes easily the dominant genus followed by _Acarospora_, _Caloplaca_ and _Lecidea_.

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

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

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

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

M. C. Knowles[1213] reports concerning the lichen flora of some mountain lakes in Waterford, that a band of _Dermatocarpon miniatum_ var. _complicatum_ six feet wide grew all the way round the lakes between the winter and summer level of the water. Below that zone _D. aquaticum_ formed another belt mingled with the moss _Fontinalis_ and several species of crustaceous lichens _Staurotheleae_, _Polyblastiae_, etc.

Bruce Fink[1214] gives as a typical “amphibious angiocarpous lichen formation” of wet rocks in Minnesota: _Dermatocarpon aquaticum_, _D. miniatum_ var. _complicatum_, _Staurothele clopima_ and _Verrucaria viridula_. These “formations,” he says, “may be seen complete in places along the shores of Vermillion Lake and less well represented at other portions of the lake shore.” Macmillan found that on the rocky shores of Lake Superior the _Dermatocarpon_ zone also occurred nearest the water.

Species with closed fruits such as Pyrenolichens, or with apothecia deeply sunk in the thallus and thus also well protected, seem to be best adapted to the aquatic life. Such in our own country are _Lecanora lacustris_, _Bacidia inundata_ and others, with a number of _Verrucariae_: _V. aethiobola_, _V. hydrela_, _V. margacea_, etc.

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

_Verrucaria aethiobola._
_Verrucaria hydrela._
_Dermatocarpon aquaticum._
_Bacidia inundata._
_Lecanora aquatica._

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

_Pterygium Kenmorensis._
_Collema fluviatile._
_Lecanora lacustris._
_Lecanora epulotica._
_Bacidia inundata._
_Rhizocarpum obscuration._
_Rhizocarpum petraeum._
_Lecidea contigua._
_Lecidea albocoerulescens._
_Dermatocarpon miniatum var. complicatum._
_Dermatocarpon aquaticum._
_Verrucaria laevata._
_Verrucaria aethiobola._
_Verrucaria margacea._

The second group of species usually inhabits damp, shaded rocks of ravines or large boulders by streams or near waterfalls. It includes species of _Collema_, _Sticta_, _Peltigera_, _Solorina_, _Pannaria_, etc., with _Opegrapha zonata_, _Porina lectissima_ and _Verrucaria nigrescens_.

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

D. LICHENS AS PIONEERS

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

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

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

Goeppert[1220] has noted as special rock-destroyers some foliaceous species, _Parmelia saxatilis_, _P. stygia_ and _P. encausta_, the underlying rock being roughened and broken up by their rhizoids. Species of _Gyrophora_ and _Sphaerophorus_ have the same disintegrating effect, so that the surface of the rock may in time lose its coherence to a depth of 2 to 4 inches. Crustaceous species such as _Lecanora polytropa_, _Candelariella vitellina_, etc., exercise an equally powerful solvent action, while underneath closely appressed growers like _Lecanora atra_ and _Acarospora smaragdula_ the stone is converted to a friable substance that can be sliced away with a knife.

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

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

Lucy E. Braun[1224] has described the successive colonization of limestone conglomerate in Cincinnati. The rock is somewhat resistant to erosion and stands out in irregular outcrops on the hillsides of the region. The first plants to gain a footing are certain crustaceous lichens, _Lecidea_ sp., _Pertusaria communis_, _Staurothele umbrina_, _Verrucaria muralis_ and _Placodium citrinum_ which occur as patches on the smoother and more exposed rock faces. With these were associated small quantities of a moss, _Grimmia apocarpa_. In the second stage of growth _Dermatocarpon miniatum_, and, to a lesser degree, a gelatinous _Omphalaria_ sp. were the most prominent plants, but mosses were more in evidence, and the next stage consisted almost exclusively of mosses and hepatics with _Peltigera canina_. A thick layer of humus was gradually built up by these plants on which Phanerogamous plants were able to flourish.

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

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