CHAPTER XIX 

 OSMOSIS AND IMBIBITION 



They shall not pass. 

 — General Petain. 



Discovery of Osmosis. — As early as 1827 Dutrochet noticed 

 the escape of zoospores from the sporangia of some of the algae 

 and accounted for it upon a physical basis. He formulated the 

 hypothesis that inside the sporangium were " water-attracting" 

 substances which increased the entrance of water to such a point 

 that the sporangium wall was stretched and then ruptured. He 

 had seen that when an animal bladder is filled with a salt or sugar 

 solution and then placed in pure water, the membrane is burst 

 by the entrance of water, and he explained (correctly) the two 

 phenomena in the same way. 



Work of Pfeffer. — Because these observations of Dutrochet 

 were the first in this subject, he is given credit as the discoverer 

 of osmosis, but since his day an enormous number of workers have 

 contributed to our knowledge of this important process. The 

 first to carry on extensive investigations of prime importance was 

 the German plant physiologist, Wm. Pfeffer (1877), who prepared 

 special membranes for the study of the subject. It will be noted 

 that one of the requirements for the successful observation of 

 osmotic pressure is a membrane which is differentially permeable, 

 i. e., which will let the solvent (water, alcohol, etc.) pass through 

 but which will not let the solute (salt, sugar, etc.) penetrate. 

 When the membrane is absolutely impermeable to the solute, it 

 is said to be semipermeable but such membranes are rare, so that 

 the term differentially permeable is preferable. This means that 

 the membrane lets the solvent penetrate much more easily than 



the solute. 



Membranes of parchment paper, bladder, and collodion are 

 all good for the study of osmosis, but Pfeffer prepared precipita- 

 tion membranes which are only slightly permeable to the solutes 

 and, for this reason, are exceptionally advantageous. His method 

 was to take a porous clay cylinder similar to those employed in 



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