562 LICHENS Tribe 24. Grctphidci, Nyl. Thallus thinly crastaceous, or hypo- phi ceo Jal, or rarely none proper; gonidial stratum consisting of gonidia (rarely chrysogonidia). Apothecia lirelline or rotuudate. Spermogones with simple sterigmata. Genera : Xylographa, Fr. ; Agyrium, Fr. ; Lithographa, Nyl. ; Graphis, Ach. ; Thelographis, Nyl. ; Hclminthocarpon, Fee ; Lcuco- grapha, Nyl. ; Opcgrapha, Ach. ; Platygrapha, Nyl. ; Stigmati- dium, Mey. ; Arthonia, Ach. ; Melaspilea, Nyl. ; Lecanactis, Eschw. ; Schizographa, Nyl. ; Glyphis, Ach. ; Chiodecton, Ach. Series VI. Pyrenodei, Nyl. Thallus peltate, or crustaceous, or hypophloeodal, or evanescent. Apothecia nucleiform, with an apical ostiole. Tribe 25. Pyrenocarpei, Nyl. Thallus various, often macular or obsolete ; gonidial system consistingof gonidia(rarely chrysogonidia), sometimes of gonimia. Apothecia pyrenodeine, often without para- physes. Spermogones with simple sterigmata or arthrosterigniata. Genera : Cora, Fr. ; Dichonema, N. ab Es. ; Normandina, Nyl. ; Endocarpon, Heclw. ; Verrucariiia, Nyl. ; Vcrrucaria, Pers. ; Thelenella, Nyl. ; TMopsis, Nyl. ; Obrysum, Wallr. ; Strigula, Fr. ; Parathelium, Nyl. ; Melanotlicca, Fee ; Trypc- thelium, Ach. ; Astrothelium, Eschw. Series VII. Pcridiodci, Nyl. Thallus thin, often wanting. Apothecia peridieine, without any ostiole. Tribe 26. Peridci, Nyl. Thallus thin, macular, or none proper. Apothecia consisting of a peridium. Spermogones (where seen) with simple sterigmata. Genera : Thclocarpon, Nyl. ; Tliclococcus, Nyl. ; Endococcus, Nyl. ; Mycoporum, Flot. Family IV. Myrianyiacei, Nyl. Thallus unstratified, entirely and equally cellulose. Fructifica tion not discrete. Tribe 1. Myriangiei, Nyl. Thallus noduloso-pulvinate. Apo thecia sublecanorine. Spermogones unknown. Genus : Myriangium, Mut. and Berk. In the Nylanderiiin arrangement, most of these tribes and genera are again divided into subtribes and subgenera, the latter being further subdivided into ections according to the affinities of the different species. Habitats and Distribution of Lichens. These two subjects are intimately related and present many interesting features which here we can only very generally notice without entering into details. 1. Habitats of Lichens. These are extremely varied, and comprehend 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 corticate 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 Lecidese). 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 corti- cole lichens may be mentioned those lignicole species which grow on decayed or decaying wood of trees and on old pales (e.g., Caliciei, various Lecidex, Xylograplia). (b) As to saxicole lichens, which occur on rocks and stones, they may be divided into two sections, viz., calcicole and calci- fugous. 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 Verrucarise), while all other saxicole 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 calcicole lichens (e.g., Lecanora Prevostii, Lecidea calcivora) have the power (through the carbonic acid received from the atmosphere) of forming minute faveoli in the rock, in which they are partially buried, (c) With respect to terricole species, some prefer peaty soil (e.g., Gladonia, Lecidea decolorans), others calcareous soil (e.g., Lecanora crassa, Lecidea decipiens), others argillaceous 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) Musdcole lichens again are such as are most frequently met with on decayed mosses and jungermannias, whether on the ground, trees, or rocks (e.g., Leptogium miiscicola, 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). With the exceptions of these last, it is to be observed that all the rest may, under different conditions of locality and climate, be found growing for the most part indiscriminately on the substrata mentioned, a normally saxicole species becoming corticole, a terricole one becoming muscicole, and vice versa. Amongst other instances of this that might be adduced, the case of Lecidea geographica, a peculiarly saxicole species, growing on the stems of Rhododendron in the Tyrolese Alps, and that of Lecidea rimdosa, a like peculiarly saxicole species, growing on the bark of trees in Germany, are especially striking. Sometimes also 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. Consequently it is apparent that in many cases lichens are quite in different 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 saxicole 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 aptitude for imbibing and retaining heat and moisture. As a rule lichens have a propensity for 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 rhndare, Endocarpon fluviatile, Verrucaria maura). Some species are entirely parasitical on other lichens (e.g., various Lccidex and Pyrenocarpei), and may be peculiar to one (e.g., Lecidea, vitellinaria) or common to several species (e.g., Habro- thalhis parmeliarum). A few, generally known as erratic species, have been met with growing unattached to any substratum (e.g., Parmelia revoluta, var. concentrica, Leca nora esculenta) ; but it seems somewhat doubtful if 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. The fact also that in numerous instances they are so indifferent to the nature of their habitats clearly shows that they do not at all depend upon the substratum for their nourishment. 2. Distribution of Lichens. 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 also and more especially by atmospherical and climatal conditions. At the same time it may safely be affirmed that their geo graphical 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 the palaeocrystic sea and in the torrid valleys of tropical climes, aa well as on
