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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.
 * Thallus in moist state more or less gelatinous. Gonidia always belonging to the Cyanophyceae, Lichinaceae, Ephebaceae, Collemaceae, Pyrenopsidaceae.
 * Thallus not gelatinous.
 * Coenogoniaceae, Lecideaceae, Cladoniaceae, Lecanoraceae, Pertusariaceae, Peltigeraceae, Stictaceae, Pannariaceae, Gyrophoraceae, Parmeliaceae, Cladoniaceae, Usneaceae.
 * (Hymenolichenes)
 * Cora, Dictyonema (incl. Laudatea), Corella (doubtfully placed here as the hymenium is unknown).

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 snow-line (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.

—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