14 



PLANT PHYSIOLOGY 



the solvent to pass but that are wholly impermeable to the 

 solutes. Such membranes, first discovered by Traube in 1867, 

 are called ''semipermeable" or ''selectively permeable" ones. 

 The semipermeable membrane that has been most studied is the 

 precipitation membrane of colloidal copper ferrocyanide, arising 



at the place of contact of a copper 

 sulphate solution with a potassium 

 ferrocyanide solution. According to 

 the opinion of many authors, 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 

 membrane. The protoplasmic lining of 

 the cell is a semipermeable membrane. 

 The study of such membranes is of 

 great importance in obtaining informa- 

 tion about the processes that take place 

 in the cell. 



Such a study was first undertaken by 

 Pfeffer in his classical "Osmotic Inves- 

 tigations" (1877). His work started a_ 

 new epoch in investigation of the prop- 

 ^ -Df «. , erties not only of semipermeable 



Fig. 5. — Pfeffer s osmom- "^ ^ 



eter. G, a vessel of water, membranes but also of water solutions 

 z, a porous cup with semi- -^ general. Pfeffer showed that these 



permeable' membrane depos- ^ 



ited in it. M, a mercury membranes readily let through water 

 manometer. Js, height of j^^^ ^re Quite impermeable tosubstauccs 



mercury column indicates tne ^ ^ 



osmotic pressure {after dissolvcd in it and that no exosmosis 

 ^ohsch). takes place through them. Therefore, 



the pressure developed by solutions contained within such mem- 

 branes is not temporary but permanent and attains considerable 

 magnitudes. A 2 per cent solution of cane sugar, for example, 

 produces an osmotic pressure equal to 102 cm. of mercury, i.e., 

 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 osmotic pressures developed even by weak solutions are 

 quite considerable. The precipitation membranes of copper 

 ferrocyanide, on the other hand, are very dehcate and easily 



