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    "source_title": "Encyclopaedia Britannica (1911)",
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    "chunk_id": "1911:discolichenes:cd4b853f2965",
    "title": "DISCOLICHENES",
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    "verified_text": "series i. coniocarpineae. the paraphyses branch and form a network (capillitium) over the asci, the capillitium and ejected spores forming a long persistent powdery mass (mazaedium). caliciaceae, cypheliaceae, sphaerophoraceae. series ii. graphidineae. apothecia seldom round, usually elongated-ellipsoidal, no capillitium. arthoniaceae, graphidiaceae, roccellaceae. series iii. cyclocarpineae, apothecium usually circular, no capillitium. a. spores usually two-celled, either with a strongly thickened cross-wall often perforated by a narrow canal or with cross-wall only slightly thickened. in the first case the spores are usually colourless, the second case always brown. buelliaceae, physciaceae. b. spores unicellular, parallel-multicellular or muriform, usually colourless, cross-walls usually thin. [alpha] thallus in moist state more or less gelatinous. gonidia always belonging to the cyanophyceae, lichinaceae, ephebaceae, collemaceae, pyrenopsidaceae. ß thallus not gelatinous. coenogoniaceae, lecideaceae, cladoniaceae, lecanoraceae, pertusariaceae, peltigeraceae, stictaceae, pannariaceae, gyrophoraceae, parmeliaceae, cladoniaceae, usneaceae. basidiolichenes (hymenolichenes) _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 naturlichen pflanzenfamilien_, teil i, abt. 1 * where full literature will be found up to 1898. m. funfstuck, \"der gegenwartige stand der flechtenkunde,\" _refer. generalvers. d. deut. bot. ges._ (1902). dual nature: j. baranetzky, \"beitrage zur kenntnis des selbststandigen lebens der flechtengonidien,\" _prings. jahrb. f. wiss. bot._ vii. (1869); e. bornet, \"recherches sur les gonidies des lichens,\" _ann. de sci. nat. bot._, 5 ser. n. 17 (1873); g. bonnier, \"recherches sur la synthese des lichens,\" _ann. de sci. nat. bot._, 7 ser. 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. moller, _uber die kultur flechtenbildender ascomyceten ohne algen_. (munster, 1887). sexuality: e. stahl, _beitrage zur entwickelungsgeschichte der flechten_ (leipzig, 1877); g. lindau, _uber anlage und entwickelung einiger flechtenapothecien_ (flora, 1888); e. baur, \"zur frage nach der sexualitat der collemaceae,\" _ber. d. deut. bot. ges._ (1898); \"uber anlage und entwicklung einiger flechtenapothecien\" (_flora_, bd. 88, 1901); \"untersuchungen uber die entwicklungsgeschichte der flechtenapothecien,\" _bot. zeit._ (1904); o. v. darbishire, \"uber 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.) footnote: [1] the _thalline margin_ (margo thallinus) is the projecting edge of a special layer of thallus, the amphithecium, round the actual apothecium; the _proper margin_ (margo proprius) is the projecting edge of the apothecium itself.",
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