410 THE COLLOIDAL STATE 



are of infinite complexity and it is not suggested that this is 

 the sole controlling factor, but that it may be a contributory 

 factor is not improbable. Many attempts have been made to 

 apply such considerations to the observed facts and of these 

 one example may be given. According to Butkewitsch,* 

 phenomena associated with the Donnan Equilibrium are hkely 

 to be fairly frequent in plant cells and to take an important 

 part in determining the distribution of diffusible ions ; the 

 role of the colloidal ions may be taken by ordinary molecular 

 disperse ions if the membrane is impermeable to them. The 

 Donnan Equilibrium may also affect the absorption of salts by 

 the root system, and the indiffusible ion may act from the 

 inside of the cell or possibly from the outside if such indiffusible 

 ions happen to be present in the soil. Experiments with 

 collodium membranes showed that silicic acid favoured the 

 transfer of diffusible ions to the other side of the membrane, and 

 it is suggested that silicic acid outside the cell may favour the 

 diffusion of phosphates into the cell and thus stimulate the 

 growth of the plant. Experiments with Zea mais, grown in 

 culture solutions containing phosphates, both in presence and 

 in absence of silicic acid, showed that when the solutions con- 

 tained only small quantities of phosphates the growth of the 

 plants was directly proportional to the amount of silicic acid 

 present. It is further suggested that the observations of 

 Hellriegel f and others, and more recently of Breazeale,^ 

 that an addition of sodium ions to the nutrient solution in- 

 creased the absorption of potassium by the plant, may be 

 attributed to the same cause ; thus if the membrane is im- 

 permeable to sodium salts these would act as non-diffusible 

 ions and so tend to favour the transfer of diffusible potassium 

 salts across the membrane. 



ENZYME ACTION OF COLLOIDS. 



Associated with this enormous development of surface 

 there is, of course, a corresponding development of surface 



* Butkewitsch ; " Biochem. Zeit.," 1925, l6l, 468. 

 t Hellriegel, Willfart, Romer, and Wimmer: "Arbeit, deut. land. 

 Ges.," 189S, 32. 



X Breazeale : " J. Agric. Res.," 1923, 26, 303. 



