Lichens

Thallus mostly crustaceous, occasionally squamulose or very rarely minutely fruticulose. The squamulose thallus is corticate above, the under surface appressed and attached to the substratum by penetrating hyphae, often effigurate at the circumference. Algal cells Protococcaceae. Apothecia well distinguished by the thalline margin; spores colourless, simple or variously septate or muriform.

Lecanora, Ochrolechia, Lecania, Haematomma and Phlyctis are cosmopolitan genera, some of them with a very large number of species; the other genera are more restricted in distribution and generally with few species.

The genus Candelariella is of uncertain position; the spores are 8 or many in the ascus and are simple or 1-septate, and not unfrequently become polarilocular as in Caloplacaceae, but there is no parietin present.

XLVI. Parmeliaceae

A very familiar family of foliose lichens. Genera and species are dorsiventral and stratose in structure, though some Cetrariae are fruticose in habit. Algal cells are Protococcaceae; in Physcidia they are Palmellae. In every case the upper surface of the thallus is corticate and generally of plectenchyma, the lower being somewhat similar, but in Heterodea and Physcidia, monotypic Australasian genera, the upper cortex is of branching hyphae parallel with the surface, the lower surface being non-corticate.

The Parmeliae are mostly provided with abundant rhizinae; in Cetrariae and Nephromopsis these are very sparingly present, while in Anzia (including Pannoparmelia) the medulla passes into a wide net-like structure of anastomosing hyphae.

In Heterodea, cyphellae occur on the under surface as in Stictaceae; and in Cetraria islandica bare patches have been described as pseudocyphellae. The latter lichen is one of the few that are of value as human food. Special aeration structures are present on the upper cortex of Parmelia aspidota.

XLVII. Usneaceae

This also is a familiar family of lichens, Usnea barbata the “bearded moss” being one of the first lichens noted and chronicled. Algal cells Protococcaceae. Structure radiate, the upright or pendulous habit characteristic of the family securing all-round illumination. Special adaptations of the cortex or of the internal tissues have been evolved to strengthen the thallus against the strains incidental to their habit of growth as they are attached in nearly all cases by one point only, by a special sheath, or by penetrating hold-fasts.

Apothecia are superficial or marginal and sometimes shortly stalked; spores are simple or variously septate.

Ramalina and Usnea, the most numerous, are cosmopolitan genera; Alectoria inhabits northern or hilly regions.

The genus Evernia, also cosmopolitan, represents a transition between foliose and fruticose types; the fronds of the two species, though strap-shaped and generally upright, are dorsiventral and stratose, the gonidia for the most part lying beneath one surface; the other (lower) surface is either white or very dark-coloured. Everniopsis, formed of thin branching strap-shaped fronds, is also dorsiventral.

A number of genera, Thamnolia, Siphula, etc. are of podetia-like structure, generally growing in swards. Several of them have been classified with Cladoniae, but they lack the double thallus. One of these, Endocena, a sterile monotypic Patagonian lichen, with stiff hollow coralloid fronds, was classified by Hue[1056] along with Siphula; recently he has transferred it to his family Polycaulionaceae[1057] based on Polycauliona regale (Placodium frustulosum Darbish.), and allied to Placodium Sect. Thamnoma[1058]. In recent studies Hue has laid most stress on thalline characters. He places the new family between “Ramalinaceae” and “Alectoriaceae.” Dactylina arctica is a common Arctic soil-lichen.

XLVIII. Caloplacaceae

In this family Zahlbruckner has included the squamulose or crustaceous lichens with colourless polarilocular spores, relegating those with more highly developed thallus or with brown spores to other families. He has also substituted the name Caloplaca for the older Placodium, the latter being, as he considers, less well defined.

Algal cells are Protococcaceae. The thallus is mostly light-coloured, generally some shade of yellow, and, with few exceptions, contains parietin, which gives a purple colour on the application of potash. The squamulose forms are closely appressed to the substratum, and have often a definite rounded outline (effigurate). The spores have a thick median septum with a loculus at each end and a connecting canal[1059].

In Blastenia the outer thalline margin is obscure or absent—though gonidia are frequently present below the hymenium. Caloplacaceae occur all over the globe; they are among the most brilliantly coloured of all lichens. Polycauliona Hue[1060] possibly belongs here: though based on thalline rather than on spore characters, one species at least has polarilocular spores.

XLIX. Teloschistaceae

Polarilocular colourless spores are the distinguishing feature of this family as of the Caloplacaceae. Algal cells Protococcaceae. The thallus of Teloschistaceae is more highly developed, being either foliose or fruticose, though never attaining to very large dimensions. The cortex of Xanthoria (foliose) is plectenchymatous, that of Teloschistes (fruticose) is fibrous. The species of both genera are yellow or greenish-yellow due to the presence of the lichen-acid parietin.

Both genera have a wide distribution over the globe, more especially in maritime regions.

L. Buelliaceae

A family of crustaceous lichens distinguished by the brown two-celled spores. Algal cells Protococcaceae. Zahlbruckner has included here Buellia and Rinodina; the former with a distinctly lecideine fruit and with thinly septate spores; the latter lecanorine and with spores of the polarilocular type, with a very wide central septum pierced in most of the species by a canal which may or may not traverse the middle lamella of the wall. Rinodina is closely allied to Physciaceae, while Buellia has more affinity with Lecideaceae and is near to Rhizocarpon.

Both genera are of world-wide distribution.

LI. Physciaceae

Thallus foliose or partly fruticose, and generally attached by rhizinae. Algal cells Protococcaceae. The spores resemble those of Rinodina, dark-coloured with a thick septum and reduced cell-lumina. As in that species there may be a second septum in each cell, giving a 3-septate spore; but that is rare.

Pyxine, a tropical or subtropical genus, is lecanorine only in the very early stages; it soon loses the thalline margin. Anaptychia is differentiated from Physcia by the subfruticose habit, though the species are nearly all dorsiventral in structure, only a few of them being truly radiate and corticate on both surfaces. The upper cortex of Anaptychia is fibrous, but that character appears also in most species of Physcia either on the upper or the lower side. Physcia and Anaptychia are widely distributed.

C. *Hymenolichens

Fungus a Basidiomycete, akin to Thelephora. Algal cells Scytonema or Chroococcus. Thallus crustaceous, squamulose or foliose. Spores colourless, produced on basidia, on the under surface of the free thallus.

The Hymenolichens[1061] are few in number and are endemic in tropical or warm countries. They inhabit soil or trees.

II. NUMBER AND DISTRIBUTION OF LICHENS

1. ESTIMATES OF NUMBER

Calculations have been made and published, once and again, as to the number of lichen species occurring over the globe or in definite areas. In 1898 Fünfstück stated that about 20,000 different species had been described, but as many of them had been proved to be synonyms, and since many must rank as forms or varieties, the number of well-authenticated species did not then, according to his estimate, exceed 4000. Many additional genera and species have, however, been discovered since then. In Engler and Prantl’s Pflanzenfamilien, over 50 families and nearly 300 genera find a place, but even in these larger groupings opinions differ as to the limits both of genera and families, and lichenologists would not all accept the arrangement given in that volume.

Fünfstück has reckoned that of his estimated 4000, about 1500 are European and of these at least 1200 occur in Germany. Probably this is too low an estimate for that large country. Leighton in 1879 listed, in his British Lichen Flora, 1710 in all, and, as the compilation includes varieties, it cannot be considered as very far astray. On comparing it with Olivier’s[1062] recent statistics of lichens, we find that of the larger fruticose and foliose species, 310 are recognized by him for the whole of Europe, 206 of these occurring in the British Isles. Leighton’s estimate of similar species is about 145, without including varieties now reckoned as good species. In a more circumscribed area, Th. Fries[1063] described for Spitzbergen about 210 different lichens, a number that closely approximates to the 206 recent records by Darbishire[1064] for the same area.

A general idea of the comparative numbers of the different types of lichens may be gathered from Hue’s compilation of exotic lichens[1065], examined or described by Nylander, and now in the Paris herbarium. There are 135 genera with 3686 species. Of these, about 829 belong to the larger foliose and fruticose lichens (including Cladoniae); the remaining 2857 belong to the smaller kinds, most of them crustaceous.

2. GEOGRAPHICAL DISTRIBUTION

A. General Survey

The larger foliose and fruticose lichens are now fairly well known and described for Europe, and the knowledge of lichens in other continents is gradually increasing. It is the smaller crustaceous forms that baffle the investigator. The distribution of all lichens over the surface of the earth is controlled by two principal factors, climate and substratum; for although lichens as a rule require only support, they are most of them restricted to one or another particular substratum, either organic or inorganic. As organisms which develop slowly, they require an unchanging substratum, and as sun-plants they avoid deeply shaded woodlands: their occurrence thus depends to a large extent on the configuration and general vegetation of the country.

Though so numerous and so widely distributed, lichens have not evolved that great variety of families and genera characteristic of the allied fungi and algae. They conform to a few leading types of structure, and thus the Orders and Families are comparatively few, and more or less universal. They are most of them undoubtedly very old plants and were probably wide-spread before continents and climates had attained their present stability. Arnold[1066] indeed considers that a large part of the present-day lichens were almost certainly already evolved at the end of the Tertiary period, and that they originated in a warm or probably subtropical climate. As proof of this he cites such genera as Graphis, Thelotrema and Arthonia[1067] which are numerous in the tropics though rare in the colder European countries; and he sees further proof in the fact that many fruticose and gelatinous lichens do not occur further north than the forest belt, though they are adapted to cold conditions. Several genera that are abundant in the tropics are represented outside these regions by only one or few species, as for instance Conotrema urceolatum and Bombyliospora incana.

During the Ice age of the Quaternary period, not many new species can have arisen, and such forms as were not killed off must have been driven towards the south. As the ice retreated the valleys were again stocked with southern forms, and northern species were left behind on mountain tops all over the globe.

In examining therefore the distribution of lichens, it will be found that the distinction between different countries is relative, certain families being more or less abundant in some regions than others, but, in general, nearly all being represented. Certain species are universal, where similar conditions prevail. This is especially true of those species adapted to extreme cold, as that condition, normal in polar regions, recurs even on the equator if the mountains reach the limit of perpetual snow; the vertical distribution thus follows on the lines of the horizontal.

In all the temperate countries we find practically the same families, with some few exceptions; there is naturally more diversity of genera and species. Genera that are limited in locality consist, as a rule, of one or few species. In this category, however, are not included the tropical families or genera which may be very rich in species: these are adapted to extreme conditions of heat and often of moisture, and cannot exist outside tropical or subtropical regions, extreme heat being more restricted as to geographical position than extreme cold.

In the study of distribution the question which arises as to the place of origin of such widely distributed plants is one that is difficult to solve. Wainio[1068] has attempted the task in regard to Cladonia, one of the most unstable genera, the variations of form, which are dependent on external circumstances, being numerous and often bewildering. In his fine monograph of the genus, 132 species are described and 25 of these are cosmopolitan.

The distribution of Phanerogams is connected, as Wainio points out, with causes anterior to the present geological era, but this cannot be the case in a genus so labile and probably so recent as Cladonia, though some of the species have existed long enough to spread and establish themselves from pole to pole. Endemic species, or those that are confined to a comparatively limited area, are easily traced to their place of origin, that being generally the locality where they are found in most abundance, and as a general rule in the centre of that area, though there may be exceptions: a plant for instance that originated on a mountain would migrate only in one direction—towards the regions of greater cold.

The difficulty of determining the primitive stations of cosmopolitan, or of widely spread, species is much greater, but generally they also may be referred to their area of greatest abundance. Thus a species may occur frequently in one continent and but rarely in another, even where the conditions of climate, etc., are largely comparable. It may therefore be inferred that the plant has not yet reached the full extent of possible distribution in the less frequented area. As examples of this, Wainio cites, among other instances, Cladonia papillaria, which has a very wide distribution in Europe, but, as yet, has been found only in the eastern parts of North America; and Cl. pycnoclada, a plant which braves the climate of Cape Horn and the Falkland Islands, but has not travelled northward beyond temperate North America: the southern origin of that species is thus plainly indicated. Wainio also finds that evidence of the primitive locality of a very widely spread species may be obtained by observing the locality of species derived from it, which are as yet of limited distribution; presumably these arose in the ancestral place of origin, though this indication is not always to be relied on. If, however, the ancestral plant has given rise to several of these rarer related species, those of them that are most closely allied to the primitive plant would be found near to it in the original locality.

A detailed account of species distribution according to these indications is given by Wainio and is full of interest. No such attempt has been made to deal with any other group, and the distribution of genera and species can only be suggested. An exhaustive comparison of the lichens of different regions is beyond the purpose of our study and is indeed impossible as, except in some limited areas, or for certain species, the occurrence and distribution are not fully known. It is in any case only tentatively that genera or species can be described as local or rare, until diligent search has been made for them over a wider field. The study of lichens from a floristic point of view lags behind that of most other groups of plants. The larger lichen forms have received more attention, as they are more evident and more easily collected; but the more minute species are not easily detected, and, as they are largely inseparable from their substratum of rocks, or trees, etc., on which they grow, they are often difficult to collect. They are also in many instances so indefinite, or so alike in outward form, that they are liable to be overlooked, only a microscopic examination revealing the differences in fruit and vegetative structure.

Though much remains to be done, still enough is known to make the geographical distribution of lichens a subject of extreme interest. It will be found most instructive to follow the usual lines of treatment, which give the three great divisions: the Polar, the Temperate and the Tropical regions of the globe.

B. Lichens of Polar Regions

Strictly speaking, this section should include only lichens growing within the Polar Circles; but in practice the lichens of the whole of Greenland and those of Iceland are included in the Arctic series, as are those of Alaska: the latitudinal line of demarcation is not closely adhered to. With the northern lichens may also be considered those of the Antarctic continent, as well as those of the islands just outside the Antarctic Circle, the South Shetlands, South Orkneys, Tierra del Fuego, South Georgia and the Falkland Islands. During the Glacial period, the polar forms must have spread with the advancing cold; as the snow and ice retreated, these forms have been left, as already stated, on the higher colder grounds, and representatives of polar species are thus to be found very far from their original haunts. There are few exclusively boreal genera: the same types occur at the Poles as in the higher subtemperate zones. One of the most definitely polar species, for instance, Usnea (Neuropogon) melaxantha grows in the whole Arctic zone, and, in the Antarctic, is more luxuriant than any other lichen, but it has also been recorded from the Andes in Chili, Bolivia and Peru, and from New Zealand (South Island).

Cold winds are a great feature of both poles, and the lichens that by structure or habit can withstand these are the most numerous; those that have a stout cortical layer are able to resist the low temperatures, or those that grow in tufts and thus secure mutual protection. In Arctic and Subarctic regions, 495 lichens have been recorded, most of them crustaceous. Among the larger forms the most frequently met are certain species of Peltigera, Parmelia, Gyrophora, Cetraria, Cladonia, Stereocaulon and Alectoria. Among smaller species Lecanora tartarea spreads everywhere, especially over other vegetation, Lecanora varia reaches the farthest limits to which wood, on which it grows, has drifted, and several species of Placodium occur constantly, though not in such great abundance. Over the rocks spread also many crustaceous Lecideaceae too numerous to mention, one of the most striking being the cosmopolitan Rhizocarpon geographicum.

Wainio[1069] has described the lichens collected by Almquist at Pitlekai in N.E. Siberia just on the borders of the Arctic Circle, and he gives a vivid account of the general topography. The snow lies on the ground till June and falls again in September, but many lichens succeed in growing and fruiting. It is a region of tundra and sand, strewn more or less with stones. Most of the sand is bare of all vegetation; but where mosses, etc., have gained a footing, there are also a fair number of lichens: Lecanora tartarea, Psoroma hypnorum, with Lecideae, Parmeliae, Cladoniae, Stereocaulon alpinum, Solorina crocea, Sphaerophorus globosus, Alectoria nigricans and Gyrophora proboscidea. Some granite rocks in that neighbourhood rise to a height of 200 ft., and though bare of vegetation on the north side, yet, in sheltered nooks, several species are to be found. Stunted bushes of willow grow here and there, and on these occur always the same species: Placodium ferrugineum, Rinodina archaea, Buellia myriocarpa and Arthopyrenia punctiformis. Some species such as Sphaerophorus globosus, Dactylina arctica (a purely Arctic genus and species) and Thamnolia vermicularis are so abundant that they bulk as largely as other better represented genera such as Cladoniae, Lecanorae or Lecideae. On the soil, Lecanorae cover the largest areas.

Wainio determined a large number of lichens with many new species, but the region is colder than that of Lappland, and trees with tree-lichens are absent, with the exception of those given above. In Arctic Siberia, Elenkin[1070] discovered a new lichen Placodium subfruticulosum which scarcely differs from Darbishire’s[1071] Antarctic species Pl. fruticulosum (or P. regale); both are distinguished by the fruticose growth of the thallus, for which reason Hue[1072] placed them in a new genus, Polycauliona.

The Antarctic Zone and the neighbouring lands are less hospitable to plant life than the northern regions, and there is practically no accumulation of detritus. Collections have been made by explorers, and several lists have been published which include a marvellous number of species common to both Poles, if the subantarctic lands are included in the survey. An analytic study of the various lists has been published by Darbishire[1073]. He recognizes 106 true Antarctic lichens half of which are Arctic as well. The greater number are crustaceous and are plants common also to other lands though a certain number are endemic. The most abundant genera in species as well as individuals are Lecidea and Lecanora. Several bright yellow species of Placodium—Pl. elegans, Pl. murorum, etc., are there as at the North Pole. Among the larger forms, Parmeliae, Cetrariae, and Cladoniae are fairly numerous; Usneae and Ramalinae rather uncommon, while members of the Stictaceae are much more abundant than in the North. The common species of Peltigera also occur in Antarctica, though P. aphthosa and P. venosa are wanting; both of these latter are boreal species. Darbishire adds that lichens have so great a capacity to withstand cold, that they are only checked by the snow covering, and were bare rocks to be found at the South Pole, he is sure lichens would take possession of them. The most southerly point at which any plant has been found is 78° South latitude and 162° East longitude, in which locality the lichen Lecanora subfusca was collected by members of Scott’s Antarctic expedition (1901-1904) at a height of 5000 ft.

A somewhat different view of the Antarctic lichen flora is indicated by Hue[1072] in his account of the plants brought back by the second French Antarctic Expedition. The collection was an extremely favourable and important one: great blocks of stone with their communities of lichens were secured, and these blocks were entirely covered, the crustaceous species, especially, spreading over every inch of space.

Hue determined 126 species, but as 15 of these came from the Magellan regions only 111 were truly Antarctic. Of these 90 are new species, 29 of them belonging to the genus Buellia. Hue considers, therefore, that in Antarctica there is a flora that, with the exception of cosmopolitan species, is different from every other, and is special to these southern regions. Darbishire himself described 34 new Antarctic species, but only 10 of these are from true Antarctica; the others were collected in South Georgia, the Falkland Islands or Tierra del Fuego. Even though many species are endemic in the south, the fact remains that a remarkable number of lichens which occur intermediately on mountain summits are common to both Polar areas.

C. Lichens of the Temperate Zones

Regions outside the Polar Circles which enjoy, on the whole, cool moist climates, are specially favourable to lichen growth, and the recorded numbers are very large. The European countries are naturally those in which the lichen flora is best known. Whereas polar and high Alpine species are stunted in growth and often sterile, those in milder localities grow and fruit well, and the more highly developed species are more frequent. Parmeliae, Nephromae, Usneae and Ramalinae become prominent, especially in the more northern districts. Many Arctic plants are represented on the higher altitudes. A comparison has been made between the lichens of Greenland and those of Germany: of 286 species recorded for the former country, 213 have been found in Germany, the largest number of species common to both countries being crustaceous. Lindsay[1074] considered that Greenland lichens were even more akin to those of Scandinavia.

There is an astonishing similarity of lichens in the Temperate Zone all round the world. Commenting on a list of Chicago lichens by Calkins[1075], Hue[1076] pointed out that with the exception of a few endemic species they resemble those of Normandy. The same result appears in Bruce Fink’s[1077] careful compilation of Minnesota lichens, which may be accepted as typical of the Eastern and Middle States of North Temperate America. The genera from that region number nearly 70, and only two of these, Omphalaria and Heppia, are absent from our British Flora. The species naturally present much greater diversity. Very few Graphideae are reported. In other States of North America there occurs the singular aquatic lichen, Hydrothyria venosa, nearly akin to Peltigera.

If we contrast American lichens with these collected in South Siberia near Lake Baikal[1078], we recognize there also the influence of temperate conditions. Several species of Usnea are listed, U. barbata, U. florida, U. hirta and U. longissima, all of them also American forms, U. longissima having been found in Wisconsin. Xanthoria parietina, an almost cosmopolitan lichen, is absent from this district, and is not recorded from Minnesota. The opinion[1079] in America is that it is a maritime species: Tuckerman gives its habitat as “the neighbourhood of a great water,” and reports it from near Lake Superior. In our country it grows at a good distance from the sea, in Yorkshire dales, etc., but all our counties would rank as maritime in the American sense. Lecanora tartarea which is rare in Minnesota is also absent from the Lake Baikal region. It occurs frequently both in Arctic and in Antarctic regions, and is probably also somewhat maritime in habitat. Many of the Parmeliae, Nephromiae and Peltigerae, common to all northern temperate climes, are Siberian as are also Cladoniae and many crustaceous species. There is only one Sticta, St. Wrightii, a Japanese lichen, recorded by Wainio from this Siberian locality.

A marked difference as regards species is noted between the Flora of Minnesota and that of California. Herre[1080] has directed attention to the great similarity between the lichens of the latter state and those of Europe: many European species occur along the coast and nowhere else in America so far as is yet known; as examples he cites, among others, Calicium hyperellum, Lecidea quernea, L. aromatica, Gyrophora polyrhiza, Pertusaria amara, Roccella fuciformis, R. fucoides and R. tinctoria. The Scandinavian lichen, Letharia vulpina, grows abundantly there and fruits freely; it is very rare in other parts of America. Herre found, however, no specimens of Cladonia rangiferina, Cl. alpestris or Cl. sylvatica, nor any species of Graphis; he is unable to explain these anomalies in distribution, but he considers that the cool equable climate is largely responsible: it is so much more like that of the milder countries of Europe than of the states east of the Sierra Nevada. His contention is supported by a consideration of Japanese lichens. With a somewhat similar climate there is a great preponderance of European forms. Out of 382 species determined by Nylander[1081], 209 were European. There were 17 Graphideae, 31 Parmeliae, and 23 Cladoniae, all of the last named being European. These results of Nylander’s accord well with a short list of 30 species from Japan compiled by Müller[1082] at an earlier date. They were chiefly crustaceous tree-lichens; but the Cladoniae recorded are the familiar British species Cl. fimbriata, Cl. pyxidata and Cl. verticillata.

With the Japanese Flora may be compared a list[1083] of Maingay’s lichens from China, 35 in all. Collema limosum, the only representative of Collemaceae in the list, is European, as are the two species of Ramalina, R. gracilenta and R. pollinaria; four species of Physcia are European, the remaining Ph. picta being a common tropical or subtropical plant. Lecanora saxicola, L. cinerea, Placodium callopismum and Pl. citrinum are cosmopolitan, other Lecanorae and most of the Lecideae are new. Graphis scripta, Opegrapha subsiderella and Arthonia cinnabarina—the few Graphideae collected—are more or less familiar home plants. Among the Pyrenocarpei, Verrucaria (Pyrenula) nitida occurs; it is a widely distributed tree-lichen.

It is unnecessary to describe in detail the British lichens. Some districts have been thoroughly worked, others have barely been touched. The flora as a whole is of a western European type showing the influence of the Gulf Stream, though there is also a representative boreal growth on the moorlands and higher hills, especially in Scotland. Such species as Parmelia pubescens, P. stygia and P. alpicola recall the Arctic Circle while Alectoriae, Cetrariae and Gyrophorae represent affinity with the colder temperate zone.

In the southern counties such species as Sticta aurata, S. damaecornis, Phaeographis Lyellii and Lecanora (Lecania) holophaea belong to the flora of the Atlantic seaboard, while in S.W. Ireland the tropical genera Leptogidium and Anthracothecium are each represented by a single species. The tropical or subtropical genus Coenogonium occurs in Great Britain and in Germany, with one sterile species, C. ebeneum. Enterographa crassa is another of our common western lichens which however has travelled eastwards as far as Wiesbaden. Roccella is essentially a maritime genus of warm climates: two species, R. fuciformis and R. fucoides, grow on our south and west coasts. The famous R. tinctoria is a Mediterranean plant, though it is recorded also from a number of localities outside that region and has been collected in Australia.

In the temperate zones of the southern hemisphere are situated the great narrowing projections of South Africa and South America with Australia and New Zealand. As we have seen, the Antarctic flora prevails more or less in the extreme southern part of America, and the similarity between the lichens of that country and those of New Zealand is very striking, especially in the fruticulose forms. There is a very abundant flora in the New Zealand islands with their cool moist climate and high mountains. Churchill Babington[1084] described the collections made by Hooker. Stirton[1085] added many species, among others Calycidium cuneatum, evidently endemic. Later, Nylander[1086] published the species already known, and Hellbom[1087] followed with an account of New Zealand lichens based on Berggsen’s collections; many more must be still undiscovered. Especially noticeable as compared with the north, are the numbers of Stictaceae which reach their highest development of species and individuals in Australasia. They are as numerous and as prominent as are Gyrophoraceae in the north. A genus of Parmeliaceae, Hetorodea, which, like the Stictae, bears cyphellae on the lower surface, is peculiar to Australia.

A warm current from the tropical Pacific Ocean passes southwards along the East Coast of Australia, and Wilson[1088] has traced its influence on the lichens of Australia and Tasmania to which countries a few tropical species of Graphis, Chiodecton and Trypethelium have migrated. Various unusual types are to be found there also: the beautiful Cladonia retepora (Fig. 71), which spreads over the ground in cushion-like growths, with the genera Thysanothecium and Neophyllis, genera of Cladoniaceae endemic in these regions.

The continent of Africa on the north and east is in so close connection with Europe and Asia that little peculiarity in the flora could be expected. In comparing small representative collections of lichens, 37 species from Egypt and 20 from Palestine, Müller[1089] found that there was a great affinity between these two countries. Of the Palestine species, eight were cosmopolitan; among the crustaceous genera, Lecanorae were the most numerous. There was no record of new genera.

The vast African continent—more especially the central region—has been but little explored in a lichenological sense; but in 1895 Stizenberger[1090] listed all of the species known, amounting to 1593, and new plants and new records have been added since that day. The familiar genera are well represented, Nephromium, Xanthoria, Physcia, Parmelia, Ramalina and Roccella, some of them by large and handsome species. In the Sahara Steiner[1091] found that genera with blue-green algae such as the Gloeolichens were particularly abundant; Heppia and Endocarpon were also frequent. Algeria has a Mediterranean Flora rather than tropical or subtropical. Flagey[1092] records no species of Graphis for the province of Constantine, and only 22 species of other Graphideae. Most of the 519 lichens listed by him there are crustaceous species. South America stretches from the Tropics in the north to Antarctica in the south. Tropical conditions prevail over the central countries and tropical tree-lichens, Graphidaceae, Thelotremaceae, etc. are frequent; further West, on the Pacific slopes, Usneae and Ramalinae hang in great festoons from the branches, while the foliose Parmeliae and Stictae grow to a large size on the trunks of the trees.

Wainio’s[1093] Lichens du Brésil is one of the classic systematic books and embodies the writer’s views on lichen classification. There are no new families recorded though a number of genera and many species are new, and, so far as is yet known, these are endemic. Many of our common forms are absent; thus Peltigera is represented by three species only, P. leptoderma, P. spuriella and P. Americana, the two latter being new species. Sticta (including Stictina) includes only five species, and Coenogonium three. There are 39 species of Parmelia with 33 of Lecanora and 68 of Lecidea, many of them new species.

D. Lichens of Tropical Regions

In the tropics lichens come under the influence of many climates: on the high mountains there is a region of perpetual snow, lower down a gradual change to temperate and finally to tropical conditions of extreme heat, and, in some instances, extreme moisture. There is thus a bewildering variety of forms. By “tropical” however the warmer climate is always implied. Several families and genera seem to flourish best in these warm moist conditions and our familiar species grow there to a large size. Among crustaceous families Thelotremaceae and Graphidaceae are especially abundant, and probably originated there. In the old comprehensive genus Graphis, 300 species were recorded from the tropics. It should be borne in mind that Trentepohlia, the alga that forms the gonidia of these lichens, is very abundant in the tropics. Coenogonium, a genus containing about twelve species and also associated with Trentepohlia, is scarcely found in Europe, except one sterile species, C. ebeneum. Other species of the genus have been recorded as far north as Algeria in the Eastern Hemisphere and Louisiana in the Western, while one species, C. implexum, occurs in the southern temperate zone in Australia and New Zealand.

Of exclusively tropical lichens, the Hymenolichens are the most noteworthy. They include three genera, Cora, Corella and Dictyonema, the few species of which grow on trees or on the ground both in eastern and western tropical countries.

Other tropical or subtropical forms are Oropogon loxensis, similar to Alectoria in form and habit, but with one brown muriform spore in the ascus; it is only found in tropical or subtropical lands. Physcidia Wrightii (Parmeliaceae) is exclusively a Cuban lichen. Several small genera of Pyrenopsidaceae such as Jenmania (British Guiana), Paulia (Polynesia) and Phloeopeccania (South Arabia) seem to be confined to very hot localities. On the other hand Collemaceae are rare: Wainio records from Brazil only four species of Collema, with nine of Leptogium.

Among Pyrenolichens, Paratheliaceae, Mycoporaceae and Astrotheliaceae are almost exclusively of tropical distribution, and finally the leaf lichens with very few exceptions. These follow the leaf algae, Mycoidea, Phycopeltis, etc., which are so abundant on the coriaceous long-lived green leaves of a number of tropical Phanerogams. All the Strigulaceae are epiphytic lichens. Phyllophthalmaria (Thelotremaceae) is also a leaf genus; one of the species, Ph. coccinea, has beautiful carmine-red apothecia. The genera of the tropical family Ectolechiaceae also inhabit leaves, but they are associated with Protococcaceae; one of the genera Sporopodium[1094] is remarkable as having hymenial gonidia. Though tropical in the main, epiphyllous lichens may spread to the regions beyond: Sporopodium Caucasium and a sterile Strigula were found by Elenkin and Woronichin[1095] on leaves of Buxus sempervirens in the Caucasus, well outside the tropics.

Pilocarpon, an epiphytic genus, is associated with Protococcaceae; one of the species, P. leucoblepharum, spreads from the bark to the leaves of pine-trees; it is widely distributed and has also been reported in the Caucasus[1096]. Chrysothrix, in which the gonidia belong to the algal genus Palmella, grows on Cactus spines in Chili, and may also rank as a subtropical epiphyllous lichen.

A series of lichens from the warm temperate region of Transcaucasia investigated by Steiner[1097] were found to be very similar to those of Central Europe. Lecanoraceae were, however, more abundant than Lecideaceae and Verrucariaceae were comparatively rare.

Much of Asia lies within tropical or subtropical influences. Several regions have received some amount of attention from collectors. From Persia there has been published a list of 59 species determined by Müller[1098]; several of them are Egyptian or Arabian plants, 15 are new species, but the greater number are European.

A small collection of 53 species from India, near to Calcutta, published by Nylander[1099], included a new genus of Caliciaceae, Pyrgidium (P. bengalense), allied to Sphinctrina. He also recorded Ramalina angulosa in African species, along with R. calicaris, R. farinacea and Parmelia perlata, f. isidiophora, which are British. Other foliose forms, Physcia picta, Pyxine Cocoës and P. Meissnerii are tropical or subtropical; along with these were collected crustaceous tropical species belonging to Lecanorae, Lecideae, Graphideae, etc.

Leighton[1100] published a collection of Ceylon lichens and found that Graphideae predominated. Nylander[1101] came to the same conclusion with regard to lichens referred to him: out of 159 species investigated from Ceylon, there were 36 species of Graphideae. In another list[1102] of Labuan, Singapore and Malacca lichens, 164 in all, he found that 56 belonged to the Graphidei, 36 to Pyrenocarpei, 14 to Thelotremei and 11 to Parmelei; only 15 species were European.

On the whole it is safe to conclude from the above and other publications that the exceptional conditions of the tropics have produced many distinctive lichens, but that a greater abundance both of species and individuals is now to be found in temperate and cold climates.

III. FOSSIL LICHENS

In pronouncing on the great antiquity of lichens, proof has been adduced from physiological rather than from phytogeological evidence. It would have been of surpassing interest to trace back these plants through the ages, even if it were never possible to assign to any definite period the first symbiosis of the fungus and alga; but among fossil plants there are only scanty records of lichens and even these few are of doubtful determination.

The reason for this is fairly obvious: not only are the primitive thalline forms too indistinct for recognizable preservation, but all lichens are characterized by the gelatinous nature of the hyphal or of the algal membranes which readily imbibe water. They thus become soft and flaccid and unfit to leave any impress on sedimentary rocks. It has also been pointed out by Schimper[1103] that while deciduous leaves with fungi on them are abundant in fossil beds, lichens are entirely wanting. These latter are so firmly attached to the rocks or trees on which they grow that they are rarely dislodged, and form no part of wind- or autumn-fall. Trunks and branches of trees lose their bark by decay long before they become fossilized and thus all trace of their lichen covering disappears.

The few records that have been made are here tabulated in chronological order:

1. Palaeozoic. Schimper decides that there are no records of lichens in the earlier epochs. Any allusions[1104] to their occurrence are held to be extremely vague and speculative.

2. Mesozoic. Braun[1105] has recorded a Ramalinites lacerus from the Keuper sandstone at Eckersdorf, though later[1106] he seemed to be doubtful as to his determination. One other lichen, an Opegrapha, has been described[1107] from the chalk at Aix.

3. Cainozoic. In the brown-coal formations of Saxony Engelhardt[1108] finds two lichens: Ramalina tertiaria, a much branched plant, the fronds being flat and not channelled “and of further interest that it is attached to a carbonized stem.” The second form, Lichen dichotomus, has a dichotomously branching strap-shaped frond. “There is sufficient evidence that these fronds were cylindrical and that the width is due to pressure. In one place a channel is visible, filled with an ochraceous yellow substance.”

Other records on brown coal or lignite are: Verrucarites geanthricis[1109] Goepp., somewhat similar to Pyrenula nitida, found at Muskau in Silesia; Opegrapha Thomasiana[1110] Goepp., near to Opegrapha varia, and Graphis scripta succinea Goepp.[1111] on a piece of lignite in amber beds, all of them doubtful.

Schimper has questioned, as he well might, Ludwig’s[1112] records from lignite from the Rhein-Wetterau Tertiary formations; these are: Cladonia rosea, Lichen albineus, L. diffissus and L. orbiculatus; he thinks they are probably fungus mycelia. Another lichen, a Parmelia with apothecia, which recalls somewhat P. saxatilis or P. conspersa, collected by Geyler also in the brown coal of Wetterau is accepted by Schimper[1113] as more trustworthy.

More authentic also are the lichens from the amber beds of Königsberg and elsewhere collected by Goeppert and others. These deposits are Cainozoic and have been described by Goeppert and Menge[1114] as middle Miocene. Schimper gives the list as: Parmelia lacunosa Meng. and Goepp., fragments of thallus near to P. saxatilis; Sphaerophorus coralloides; Cladonia divaricata Meng. and Goepp.; Cl. furcata; Ramalina calicaris vars. fraxinea and canaliculata; Cornicularia aculeata, C. subpubescens Goepp., C. ochroleuca, C. succinea Goepp., and Usnea barbata var. hirta. Schimper rather deprecates specific determinations when dealing with such imperfect fragments.

In a later work Goeppert and Menge[1114] state that they have found twelve different amber lichens and that among these are Physcia ciliaris, Parmelia physodes and Graphis (probably G. scripta succinea) along with Peziza resinae which is more generally classified among lichens as Lecidea (Biatorella) resinae.

Another series of lichens found in recent deposits in North Europe has been described by Sernander[1115] as “subfossil.” While engaged on the investigations undertaken by the Swedish Turf-Moor Commission, he noted the alternation of slightly raised Sphagnum beds with lower-lying stretches of Calluna and lichen moor—in some instances dense communities of Cladonia rangiferina. In time the turf-forming Sphagnum overtopped and invaded the drier moorland, covering it with a new formation of turf. Beneath these layers of “regenerated turf” were found local accumulations of blackened remains of the Cladonia still recognizable by the form and branching. Some specimens of Cetraria islandica were also determined.

Of especial lichenological interest in these northern regions was the Calcareous Tufa or Calc-sinter in which Sernander also found subfossil lichens—distinct impressions of Peltigera spp. and the foveolae of endolithic calcicolous species.

In another category he has placed Ramalina fraxinea, Graphis sp. and Opegrapha sp., traces of which were embedded with drift in the Tufa. In the two Graphideae the walls of apothecia and pycnidia were preserved. Sernander considers their presence of interest as testifying to warmer conditions than now prevail in these latitudes.

CHAPTER IX
ECOLOGY

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.