110 TEXTBOOK OF PLANT PHYSIOLOGY 



ous membranes which only slightly check the diffusion of water 

 and its solutes, there exist membranes which allow water or the 

 solvent to pass, but which are wholly impermeable to the solutes. 

 Such membranes, first discovered by Traube, in 1867, have been 

 called semipermeable or selectively permeable ones. The semi- 

 permeable membrane which has been most studied is the precipi- 

 tation membrane of colloidal copper ferrocyanide, arising at the 

 place of contact of a copper sulphate solution with a potassium 

 ferrocyanide solution. The penetration of substances through 

 such membranes takes place not by diffusion through the pores 

 of the membrane, but by solution of the substances in the mem- 

 brane. Only such penetration in which the primary role is played 

 not by the size of the particles of the moving substance, but by its 

 solubility in the membrane, is real osmosis. In Dutrochet's osmom- 

 eter only diffusion takes place, slightly modified by the friction of 

 the particles against the walls of the pores. The protoplasmic 

 lining of the cell is a semipermeable membrane. The study of 

 such membranes is of great importance in obtaining information 

 on the processes that take place in the cell. This study was first 

 undertaken by Pfeffer in his classical "Osmotic Investigations" 

 (1877). His work started a new epoch in the study of the proper- 

 ties not only of semipermeable membranes but also of water solu- 

 tions in general. Pfeffer showed that these membranes readily let 

 through water but are quite impermeable to the substances dis- 

 solved in it, and that no exosmosis takes place through them. 

 Therefore, the pressures developed by solutions contained within 

 such membranes are not temporary but permanent, and attain 

 considerable magnitudes. Thus, a 2 per cent solution of cane 

 sugar produces an osmotic pressure equal to 102 cm. of mercury 

 above atmospheric pressure, while in Dutrochet's osmometer even 

 with very concentrated solutions, a pressure of 10 to 15 cm. is 

 obtained with difficulty. 



The pressures developed by weak solutions are quite consider- 

 able. The precipitation membranes of copper ferrocyanide, on 

 the other hand, are very delicate and easily ruptured. Hence, in 

 measuring these pressures Pfeffer proceeded in the following way: 

 he took a porous porcelain cylinder such as is used in batteries, 

 filled it with a solution of copper sulphate, and submerged it in a 

 solution of potassium ferrocyanide. A membrane was precip- 

 itated in the small pores of the porcelain, the walls of these pores 



