Page:History of botany (Sachs; Garnsey).djvu/530

must always belong the merit of having brought into notice this mechanical effect of endosmose and of employing it to explain a number of vital phenomena. Many things in which a mechanical explanation had not been hitherto thought of could now be traced to a mechanical principle, the effects of which could be exhibited and more accurately studied by means of artificial apparatus. Dutrochet rightly attached a special value to the fact, that all states of tension in vegetable tissue could be at once explained by endosmose and exosmose, though, as so often happens in such matters, he may have extended his new principle to cases where it was not applicable, as we shall see below. His account of the nature of endosmose itself must now be considered to be obsolete, nor did the mathematician Poisson or the physicist Magnus about 1830 succeed in framing a satisfactory theory on the subject. It was discovered in the course of the succeeding twenty or thirty years, that the phenomena observed by Dutrochet, and which he called endosmose and exosmose, were only complicated cases of hydro-diffusion, which with the diffusion of gas forms an important part of molecular physics. Dutrochet, like his immediate successors, conducted his investigations into osmose with animal and vegetable membranes, the latter being of a complex structure; with these he always observed in addition to the endosmotic flow of water into the more concentrated solution, an escape of the solution itself, and from this he concluded that there must always be two currents in opposite directions through the membrane which separates the two fluids, that, as he expresses it, the endosmose is always accompanied with exosmose. This error, which was even developed later into a theory of the endosmotic equivalent, has had much to do till recently with making it impossible or difficult to refer certain phenomena of vegetation to the processes of hydro-diffusion. To mention only one case, Schleiden rightly observed that if endosmose, as Dutrochet understood it, is the sole cause why water is absorbed by the roots,