=117. The chemical nature of soils.= The effect of alkaline and acid substances in the soil upon water-content and the activities of the plant is far from being well understood. It is generally recognized that salts and acids tend to inhibit the absorptive power of the root-hairs. In the case of saline soils, this inhibitive effect seems to be established, but the action of acids in bogs and swamps is still an open question. It is probable that the influence of organic acid has been overestimated, and that the curious anomaly of a structural xerophyte in a swamp is to be explained by the stability of the ancestral type and by the law of extremes. Apart from the effect which excessive amounts of acids and salts may have in reducing the chresard, the chemical character of the soil is powerless to produce structural modification in the plant. Since Thurmann’s researches there has been no real support of the contention that the chemical properties of the soil, not its physical nature, are the decisive factors in the distribution and adaptation of plants. It is not sufficient that the vegetation of a silicious soil differs from that of a calcareous one. A soil can modify the plants upon it only though its water-content, or the solutes it contains. Hence, the chemical composition of the original rock is immaterial, except in so far as it modifies these two factors. Humus, moreover, while an important factor in growth, has no formative influence beyond that which it exerts through water-content. PHYSIOGRAPHY