But now suppose the following new factor to come in. Suppose that calcium sulphate undergoes decomposition in some one of the internal cells of the system of absorbing cells, or that it is even merely crystallised out in such a cell, or in any other way removed from solution (_e.g._ by deposition in cell-walls). This alters the state of affairs considerably. The separation of the molecules from the sap-solution is itself a cause for the flow of more of the solution to the cell concerned, and such causes of diffusion are very common in the plant. The importance of this principle consists in that it lies at the base of the whole question of selective absorption, application of manures, and the rotation of crops; and those who are acquainted with the excellent analytical results of De Saussure, Boussingault, Wolff, Trinchinetti, Gödechen, etc., and the water-culture experiments of Sachs, Nobbe, and others, will understand what an illuminating effect on these points was produced by the above generalisation, which we owe especially to Pfeffer's splendid researches into the nature of osmotic phenomena. It will now be clear, I hope, why we regard the living root-hairs as instruments--as pieces of living machinery--for the active absorption of water, with substances dissolved in it, from the soil; and it will also be evident, I think, that no one can form a proper conception of this matter of absorption, so important in all agricultural questions, unless he pays attention to these biological phenomena. It was hopeless to expect to understand these matters merely in the light of chemical analyses of plants and soils, and one expression of this hopelessness was the belief in the power of roots to select only the substances useful to it. We now know that the expression "selective power of roots" has a totally different meaning from that implied in the minds of the last generation of agriculturalists, and it would be easy to devise experiments, with solutions of different strength, where the plant should be made to take up relatively large quantities of harmless, but useless minerals, etc., and to starve in the midst of plenty of the elements proper to its structure, simply because the former are offered in a form in which they easily traverse the protoplasm of the root-hairs, while the latter are presented in a form unsuitable for absorption. That all these matters are of importance in regard to manuring and choice of soils, etc., needs no emphasising. These remarks, of course, do not detract from the value of good comparative chemical analyses, when viewed in the light of physiological knowledge, as I need hardly say; but they do, and emphatically so, attack the position that such analyses alone can explain the problems of agriculture.