a. Soil-formers. The part played by lichens in the “Economy of Nature” is of very real importance: to them is allotted the pioneer work of breaking down the hard rock surfaces and preparing a soil on which more highly developed plants can grow. This was pointed out by Linnaeus[1218] who thus describes the succession of plants: “Crustaceous lichens,” he writes, “are the first foundation of vegetation. Though hitherto we have considered theirs a trifling place among plants, nevertheless they are of great importance at that first stage in the economy of nature. When the rocks emerge from the seas, they are so polished by the force of the waves, that scarcely any kind of plant could settle on them, seen more especially near the sea. But very soon, in truth, the smallest crustaceous lichens begin to cover those arid rocks, and are sustained by minute quantities of soil and by imperceptible particles brought to them by rain and by the atmosphere. These lichens in time become converted by decay into a thin layer of humus, so that at length imbricate lichens are able to thrust their rhizoids into it. As these in turn change to humus by natural decay, various mosses such as Hypnum, Bryum and Polytrichum follow, and find suitable place and nourishment. In time there is produced by the dying down of the mosses such a quantity of soil that herbs and shrubs are able to establish themselves and maintain their existence.”
Similar observations have been made since Linnaeus’s day, among others by Guembel[1219] in his account of Lecanora ventosa. Either by the excretion of carbon dioxide which acidifies the surrounding moisture, or by the mechanical action of hyphae and rhizinae, the component particles of rocks such as granite are gradually dissolved and broken up. Rocks exposed to weather alone are unchanged, while those covered with lichens have their surface disintegrated and destroyed.
The decaying parts of the lichen thallus add to the soil material as observed by Linnaeus, and in time mosses follow, and, later, phanerogams. Goeppert[1220] has pointed out the succession observed on roofs of houses as: “first some lichen such as Lecanora saxicola, then the moss Grimmia pulvinata, which forms compact cushions on which later grow Poa compressa, small crucifers, etc.”
Goeppert[1220] has noted as special rock-destroyers some foliaceous species, Parmelia saxatilis, P. stygia and P. encausta, the underlying rock being roughened and broken up by their rhizoids. Species of Gyrophora and Sphaerophorus have the same disintegrating effect, so that the surface of the rock may in time lose its coherence to a depth of 2 to 4 inches. Crustaceous species such as Lecanora polytropa, Candelariella vitellina, etc., exercise an equally powerful solvent action, while underneath closely appressed growers like Lecanora atra and Acarospora smaragdula the stone is converted to a friable substance that can be sliced away with a knife.
Salter[1221] concluded that oxalic acid was the principal agent in disintegration. He found that it acted more or less rapidly on minerals and almost any class of saline compounds; it even attacked glass finely powdered, though silica remained unchanged.
Bachmann[1222] found that granite was reduced by lichens to a clay-like granular yellow mass in a comparatively short time, the lichen seizing on the particles of mica first; but the spread of the lichen over the rock, he observes, is largely directed by the amount of humidity and by the chance of gaining a foothold. In the case of calcareous rocks he[1223] tested the relative dampness of those containing lichens and those that were lichen-free. In the former case water was absorbed more freely and retained much longer than in the barren rock, thus encouraging further vegetation.
Lucy E. Braun[1224] has described the successive colonization of limestone conglomerate in Cincinnati. The rock is somewhat resistant to erosion and stands out in irregular outcrops on the hillsides of the region. The first plants to gain a footing are certain crustaceous lichens, Lecidea sp., Pertusaria communis, Staurothele umbrina, Verrucaria muralis and Placodium citrinum which occur as patches on the smoother and more exposed rock faces. With these were associated small quantities of a moss, Grimmia apocarpa. In the second stage of growth Dermatocarpon miniatum, and, to a lesser degree, a gelatinous Omphalaria sp. were the most prominent plants, but mosses were more in evidence, and the next stage consisted almost exclusively of mosses and hepatics with Peltigera canina. A thick layer of humus was gradually built up by these plants on which Phanerogamous plants were able to flourish.
In tropical countries the first vegetation to settle on bare rocks would seem to be blue-green gelatinous algae. Three years after the eruption of Krakatoa, dark-green layers of these plants were found by Treub[1225] on the surface of the pumice and ash, and on the loose stones in the ravines of the mountain. It was only at a later stage that lichens appeared.
b. Outposts of vegetation. Lichens are the only plants that can survive extreme conditions of cold or of heat. They grow in Polar regions where no other vegetation could obtain sustenance; they are to be found at great heights on mountains all over the globe; and, on arid desert rocks they persist through long dry seasons, depending almost entirely on night dews for the supply of moisture. Here we have true lichen formations in the sense of modern ecology.