In a special series of experiments almost all carried out in Wilhelm Pfeffer’s botanical laboratory at Leipzig, an attempt has been made to discover in what manner the cells of plants are able to withstand very high abnormalities of the osmotic pressure of the medium--that is to say, very great changes in the amount of its salinity. That many, particularly the lower plants, are able to stand such changes had been ascertained already by the careful examinations of Eschenhagen; but recent years have given us a more profound insight into what happens. Von Mayenburg[95] has found that sundry of the species of *Aspergillus*, the common mould, are able to live in very highly concentrated solutions of several salts (KNO_3 and Na_2SO_4). They were found to regulate their osmotic pressure not by taking in the salts themselves, but by raising the osmotic pressure of their own cell sap, producing a certain amount of osmotically active substances, probably carbohydrates. If in this case it were possible to assume that the osmotic pressure of the medium were the real stimulus for the production of the osmotic substances in the cell, stimulus and production both corresponding in their degree, we should be entitled to speak of a primary though physiological[96] regulation only; and it seems to me that despite the discoveries of Nathansohn that certain algae and cells of higher plants are able to change the permeability of their surfaces in a way which regulates the distribution of single salts or ions in the sap of their cells without any regard to pure osmotic equilibrium, such a simple explanation might be possible.[97] [95] *Jahrb. wiss. Bot.* 36, 1901. [96] Carbohydrates cannot be ionised, and therefore there is no doubt that in von Mayenburg’s experiments the organism itself is actively at work. As to compounds liable to ionisation, it has been noticed by Maillard that a certain regulatory character is contained simply in the physical fact that the degree of ionisation changes with concentration: decrease of concentration for instance would be followed by an increase of ionisation, and so the osmotic pressure may be preserved (*C. rend. Soc. Biol.* 53, 1901, p. 880).