_Cora_, _Dictyonema_ (incl. Laudatea), _Corella_ (doubtfully placed here
as the hymenium is unknown).
_Habitats and Distribution of Lichens._
1. _Habitats._--These are extremely varied, and comprise a great number
of very different substrata. Chiefly, however, they are the bark of
trees, rocks, the ground, mosses and, rarely, perennial leaves. (a) With
respect to _corticolous_ lichens, some prefer the rugged bark of old
trees (e.g. _Ramalina_, _Parmelia_, _Stictei_) and others the smooth
bark of young trees and shrubs (e.g. _Graphidei_ and some _Lecideae_).
Many are found principally in large forests (e.g. _Usnea_, _Alectoria
jubata_); while a few occur more especially on trees by roadsides (e.g.
_Physcia parietina_ and _Ph. pulverulenta_). In connexion with
corticolous lichens may be mentioned those _lignicole_ species which
grow on decayed, or decaying wood of trees and on old pales (e.g.
_Caliciei_, various _Lecideae_, _Xylographa_), (b) As to _saxicolous_
lichens, which occur on rocks and stones, they may be divided into two
sections, viz. _calcicolous_ and _calcifugous_. To the former belong
such as are found on calcareous and cretaceous rocks, and the mortar of
walls (e.g. _Lecanora calcarea_, _Lecidea calcivora_ and several
_Verrucariae_), while all other saxicolous lichens may be regarded as
belonging to the latter, whatever may be the mineralogical character of
the substratum. It is here worthy of notice that the apothecia of
several calcicolous lichens (e.g. _Lecanora Prevostii_, _Lecidea
calcivora_) have the power of forming minute cavities in the rock, in
which they are partially buried. (c) With respect to terrestrial
species, some prefer peaty soil (e.g. _Cladonia_, _Lecidea decolorans_),
others calcareous soil (e.g. _Lecanora crassa_, _Lecidea decipiens_),
others sandy soil or hardened mud (e.g. _Collema limosum_, _Peltidea
venosa_); while many may be found growing on all kinds of soil, from the
sands of the sea-shore to the granitic detritus of lofty mountains, with
the exception of course of cultivated ground, there being no agrarian
lichens. (d) _Muscicolous_ lichens again are such as are most frequently
met with on decayed mosses and _Jungermannia_, whether on the ground,
trees or rocks (e.g. _Leptogium muscicola_, _Gomphillus calicioides_).
(e) The _epiphyllous_ species are very peculiar as occurring upon
perennial leaves of certain trees and shrubs, whose vitality is not at
all affected by their presence as it is by that of fungi. In so far,
however, as is known, they are very limited in number (e.g. _Lecidea_,
_Bouteillei_, _Strigula_).
Sometimes various lichens occur abnormally in such unexpected habitats
as dried dung of sheep, bleached bones of reindeer and whales, old
leather, iron and glass, in districts where the species are abundant. It
is apparent that in many cases lichens are quite indifferent to the
substrata on which they occur, whence we infer that the preference of
several for certain substrata depends upon the temperature of the
locality or that of the special habitat. Thus in the case of saxicolous
lichens the mineralogical character of the rock has of itself little or
no influence upon lichen growth, which is influenced more especially and
directly by their physical properties, such as their capacity for
retaining heat and moisture. As a rule lichens grow commonly in open
exposed habitats, though some are found only or chiefly in shady
situations; while, as already observed, scarcely any occur where the
atmosphere is impregnated with smoke. Many species also prefer growing
in moist places by streams, lakes and the sea, though very few are
normally and probably none entirely, _aquatic_, being always at certain
seasons exposed for a longer or shorter period to the atmosphere (e.g.
_Lichina_, _Leptogium rivulare_, _Endocarpon fluviatile_, _Verrucaria
maura_). Some species are entirely parasitical on other lichens (e.g.
various _Lecideae_ and _Pyrenocarpei_), and may be peculiar to one (e.g.
_Lecidea vitellinaria_) or common to several species (e.g. _Habrothallus
parmeliarum_). A few, generally known as _erratic_ species, have been
met with growing unattached to any substratum (e.g. _Parmella revoluta_,
var. _concentrica_, _Lecanora esculenta_); but it can hardly be that
these are really free _ab initio_ (_vide_ Crombie in _Journ. Bot._,
1872, p. 306). It is to the different characters of the stations they
occupy with respect to exposure, moisture, &c., that the variability
observed in many types of lichens is to be attributed.
2. _Distribution._--From what has now been said it will readily be
inferred that the distribution of lichens over the surface of the globe
is regulated, not only by the presence of suitable substrata, but more
especially by climatic conditions. At the same time it may safely be
affirmed that their geographical range is more extended than that of any
other class of plants, occurring as they do in the coldest and warmest
regions--on the dreary shores of arctic and antarctic seas and in the
torrid valleys of tropical climes, as well as on the greatest mountain
elevations yet attained by man, on projecting rocks even far above the
snowline (e.g. _Lecidea geographica_). In arctic regions lichens form by
far the largest portion of the vegetation, occurring everywhere on the
ground and on rocks, and fruiting freely; while terrestrial species of
_Cladonia_ and _Stereocaulon_ are seen in the greatest luxuriance and
abundance spreading over extensive tracts almost to the entire exclusion
of other vegetation. The lichen flora of temperate regions again is
essentially distinguished from the preceding by the frequency of
corticolous species belonging to _Lecanora_, _Lecidea_ and _Graphidei_.
In intertropical regions lichens attain their maximum development (and
beauty) in the foliaceous _Stictei_ and _Parmeliei_, while they are
especially characterized by epiphyllous species, as _Strigula_, and by
many peculiar corticole _Thelotremei_, _Graphidei_ and _Pyrenocarpei_.
Some lichens, especially saxicolous ones, seem to be cosmopolitan (e.g.
_Lecanora subfusca_, _Cladonia pyxidata_); and others, not strictly
cosmopolitan, have been observed in regions widely apart. A considerable
number of species, European and exotic, seem to be _endemic_, but
further research will no doubt show that most of them occur in other
climatic regions similar to those in which they have hitherto alone been
detected. To give any detailed account, however, of the distribution of
the different genera (not to speak of that of individual species) of
lichens would necessarily far exceed available limits.
BIBLIOGRAPHY.--General: Engler and Prantl, _Die natürlichen
Pflanzenfamilien_, Teil I, Abt. 1 * where full literature will be
found up to 1898. M. Funfstuck, "Der gegenwärtige Stand der
Flechtenkunde," _Refer. Generalvers. d. deut. bot. Ges._ (1902). Dual
Nature: J. Baranetzky, "Beiträge zur Kenntnis des selbstständigen
Lebens der Flechtengonidien," _Prings. Jahrb. f. wiss. Bot._ vii.
(1869); E. Bornet, "Recherches sur les gonidies des lichens," _Ann. de
sci. nat. bot._, 5 sér. n. 17 (1873); G. Bonnier, "Recherches sur la
synthèse des lichens," _Ann. de sci. nat. bot._, 7 sér. n. 9 (1889);
A. Famintzin and J. Baranetzky, "Zur Entwicklungsgeschichte der
Gonidien u. Zoosporenbildung der Lichenen," _Bot. Zeit._ (1867, p.
189, 1868, p. 169); S. Schwendener, _Die Algentypen der
Flechtengonidien_ (Basel, 1869); A. Möller, _Über die Kultur
flechtenbildender Ascomyceten ohne Algen_. (Münster, 1887). Sexuality:
E. Stahl, _Beiträge zur Entwickelungsgeschichte der Flechten_
(Leipzig, 1877); G. Lindau, _Über Anlage und Entwickelung einiger
Flechtenapothecien_ (Flora, 1888); E. Baur, "Zur Frage nach der
Sexualität der Collemaceae," _Ber. d. deut. bot. Ges._ (1898); "Über
Anlage und Entwicklung einiger Flechtenapothecien" (_Flora_, Bd. 88,
1901); "Untersuchungen über die Entwicklungsgeschichte der
Flechtenapothecien," _Bot. Zeit._ (1904); O. V. Darbishire, "Über die
Apothecium-entwickelung der Flechte, Physcia pulverulenta," _Nyl.
Prings. Jahrb._ (Bd. 34, 1900). Chemistry.--W. Zopf, "Vergleichende
Produkte," _Beitr. z. bot. Centralbl._ (Bd. 14, 1903); _Die
Flechtenstoffe_ (Jena, 1907). (J. M. C; V. H. B.)