The same kinds of lichens occur all over these northern regions. Birger Nilson[1203] gives as the principal earth-lichens in Swedish Lappland, Alectoria ochroleuca, A. nigricans, Cetraria nivalis, C. cucullata, Cladonia uncialis, Thamnolia (Cerania) vermicularis and Sphaerophorus coralloides.
Darbishire[1204] speaks of the extensive beds of various species of Cetraria in Ellesmere Land and King Oscar Land. Alectoria nigricans and A. ochrolenca were often found in pure communities, but even more frequently in close company with mosses. Though these fruticose lichens are not represented by many species in Arctic regions, they cover a very extensive area and form a very important feature in the vegetation.
Crustaceous lichens are not wanting: Lecanora tartarea f. frigida, L. epibryon and others are to be found in great sheets covering the mosses or the soil, or spreading over the stones and boulders. Cold has no deterrent effect, and their advance is only checked by the presence of perpetual snow.
e. Desert Lichens. The reduced rainfall of desert countries is unfavourable to general lichen growth and only the more xerophytic species—those with a stout cortex—can flourish in the adverse conditions of excessive light and dryness. Lichens, however, there are, in great numbers as far as individuals are concerned, though the variety is not great. The abundance of the crustaceous Lecanora esculenta in the deserts of Asia has already been noted. Flagey[1205] found it one of the dominant species at Biskra in the Sahara where it grows on the rocks. Patouillard[1206] in describing the flora of Tunis speaks of the great patches (societies) of Lecanora crassa f. deserti which at a distance look like milk spilled on the ground, or if growing on unequal surfaces take the aspect of plaster that has been passed over by some wheeled vehicle. At Biskra species of Heppia grow on the sand. Steiner[1207] also records the frequency of Heppia and of Endocarpon in the Sahara as well as of Gloeolichens which, as they are associated with gelatinous blue-green algae, can endure extreme and long-continued desiccation. These lichens, however, only form communities in clefts among the rocks where these abut on the desert. In the great plains the sand is too mobile and too often shifted by the sirocco to enable them to settle.
Bruce Fink[1208] discusses desert lichens and their adaptive characters: crustaceous species with a stout cortex are best able to withstand the long dry periods; conspicuously lobed thalli are lacking, as are lichens with fruticose structure though he thinks the latter are prevented from developing by the exposure to high winds and driving sand storms. Herre’s[1209] study of the desert lichen flora at Reno, Nevada, is full of interest. The district is situated at an altitude of 4500 feet east of the Sierra Nevada Mountains. The annual rainfall averages 8·21 inches, and a large part falls as snow during the winter months or as early spring rain. The summer is hot and dry and the diurnal changes of temperature are very great. Strong drying winds from the west or north are frequent.
At 5000 feet and upwards lichens are, in general, exceedingly abundant on all rock substrata and represent 57 species or subspecies, only three of these being arboreal: Buellia triphragmia occurs rarely, Xanthoria polycarpa is frequent on sage brush, while Candelariella cerinella though a rock-lichen grows occasionally on the same substratum. Caloplaca (Placodium) elegans is one of the most successful and abundant species and along with Lecanora (nine forms), Acarospora (seven forms) and Lecidia (five forms) comprises three-fourths of the rock surface occupied by lichens. The addition of Rinodina with two species and Gyrophora with four brings the computation of individuals in these desert rock formations up to nine-tenths of the whole. As the desert rocks pass to the Alpine, Gyrophora becomes easily the dominant genus followed by Acarospora, Caloplaca and Lecidea.
“The colouring characteristic of the rock ledges of the desert and cañon walls is often entirely due to lichens, and in a general way they form the only brilliant plant formations in a landscape notable for its subdued pale monotonous tones. Most conspicuous are Acarospora chlorophana and Caloplaca elegans, which form striking landmarks when covering great crags and rock walls. The next most conspicuous lichens are Rinodina oreina and Lecanora rubina and its allies, which often entirely cover immense boulders and northerly sloping rock walls.” Herre concludes that though desert conditions are unfavourable to most species of lichens, yet some are perfectly at home there and the rocks are just as thickly covered as in regions of greater humidity and less sunshine.
f. Aquatic Lichens. There is only one of the larger lichens that has acquired a purely aquatic habit, Hydrothyria venosa, a North American plant. It grows on rocks[1210] in the beds of streams, covering them often with a thick felt; it is attached at the base and the rather narrow fronds float freely in the current. The gonidium is Nostoc sp., and the thallus is of a bluish-grey colour; the fruits are small discoid reddish apothecia with an evanescent margin. It is closely allied to Peltigerae, some of which are moisture-loving though not truly aquatic.
The nearest approach to aquatic habit among the foliose forms in our country is Dermatocarpon aquaticum, with thick coriaceous rather contorted lobes; it inhabits rocks and stones in streams and lakes. Somewhat less continuously aquatic is D. miniatum var. complicatum which grows on damp rocks exposed to spray or occasionally to inundation. Lindsay[1211] has described it “on boulders by the side of the Tay, frequently covered by the river when flooded, and of a deep olive colour when under water”: both these lichens have a wide distribution in Europe, Africa, America and New Zealand.
In a discussion of lake shore plants Conway Macmillan[1212] describes on the flat shores a Dermatocarpon zone on the wet area nearest the lake, behind that a Biatora zone and further landward a Cladonia zone. On rounded rocky shores the same zones followed each other but were less broad: they were so close together that the Cladoniae, which with Stereocaulon paschale grow in profusion on all such shores, occurred within a couple of feet of the high-water mark.