Every soil which bears plants contains also in its composition all the substances required by all plants, only the proportions differ, and the predominance of silex, lime, or common salt must consequently favor especially the growth of grasses, pulses, or shore-plants, although these are by no means exclusively confined to the proper sandy or calcareous soils, or to the seaside. In addition to the chemical conditions, there is yet another which modifies the former and, where it brings about the same actions, contributes to chain particular plants so much the more firmly, exclusively to particular soils, or contrariwise also contributes to conceal or obliterate the connection between plants and the chemical nature of the soil. This consists in the mechanical condition and physical peculiarities of the soil. There are plants which will only settle on unbroken _rocks_, which when the other conditions coincide, spring from these rocks over on to our _walls_, like the Wall Rue Spleenwort,[4] a little fern, the name of which denotes its station. Others occur only where weathering has broken up the solid rock into small fragments, _drift_ plants, which, clinging to mankind, select _rubbish heaps_, which most resemble their natural station; our great nettle and henbane may serve as examples. Lastly, other plants grow only where the rocks have been reduced to fine powder, in _sand_ or in the fine-grained _clay_ produced by chemical decomposition. The so-called German Sarsaparilla, the sea-reed, is an example of the first condition, but there is no definite condition corresponding to it in the vicinity of human habitations. Clay, on the other hand, stands beside the black substance humus, resulting from the decomposition of organic matter. Both rich in soluble salts, important to vegetation, both distinguished in regard to their property of absorbing from the atmosphere, and thus conveying to the roots of plants gases and aqueous vapor, they cause, singly or in combination, the most luxuriant vegetation. We thus obtain three stages in reference to the qualities of the soil-pure earths, wholly devoid of vegetation; mixed earths, without clay or humus, with an arid but characteristic vegetation; and lastly, soil rich in clay and humus, with the greatest abundance and variety of plants.