When by the action of the lichens a certain depth of loosened rock or half-formed soil has been produced, the next step is usually the advent of various mosses, which gradually shade out the crust-like lichens, while the erect kinds persist for some time. Eventually the mosses, after having increased still farther the soil layer on the rock surface, are themselves partially or wholly displaced by the hardier species of ferns; and with these the higher flowering plants, such as the stonecrops and saxifrages (the latter deriving their name from their “rock-breaking” effect), the heather, and many other or shallow-rooted plants, gradually take possession. The roots of all plants secrete carbonic acid; and many of them, much stronger vegetable acids, such as oxalic and citric. In the crevices of rocks we commonly find the roots forming a dense network over the surfaces, the marks of which show plainly the solvent effect produced on the rock by the root secretions. This is most readily observable on a polished marble surface, or on feldspathic rocks. Of course the progress of soil-formation is very much more rapid when, as in the case of powdered lava (volcanic ash) and rock debris resulting from the effects of frost etc., the surface is very much increased. In tropical climates, where both vegetative and chemical action is most intense, it takes some of the higher plants only a few years after a volcanic eruption to take possession of portions of the “ash” surfaces; thus helping to form a soil on which after a few more years agricultural plants such as the vine and olive yield paying returns. To this direct action of the higher plants is always added, to a greater or less extent, that of innumerable bacteria, as well as molds; whose vegetative and secretory action materially assists that of the roots, and the weathering process in general. _Humification._—While the mechanical action of the roots and the chemical effect of the acids of their root secretions are very efficient in promoting the transformation of mere rock powder into soil material proper, the efficacy does not end with the life of the plant. In the natural process of decay to which the roots are subject after death, and which also affects the leaves, twigs and trunks falling on the surface, the vegetable matter suffers a transformation which must be considered more in detail hereafter, and results in the formation of the complex mixture of dark-tinted substances known as vegetable mold or _humus_; the remnant of vegetation that imparts to surface soils their distinctive dark tint. Its functions in soils are both numerous, and important to vegetable growth; as regards soil formation, it assists disintegration of the rock minerals both by the formation of certain fixed, soluble acids capable of acting on them with considerable energy, and by the slow but continuous evolution of carbonic acid under the influence of atmospheric oxygen, which has been alluded to above.