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TEXTBOOK OF BOTANY 



be plasmolyzed, and the process is referred to as plasmolysis. The 

 shrunken cell is flaccid, and conversely the swollen cell is turgid. Cell 

 turgor may result from the entrance of water by imbibition or osmosis, 

 or both. Other conditions being equivalent, turgor becomes greatest in 

 the cells with the least extensible walls and in which the volume changes 



least. 



Permeability. Permeability is a property of 

 membranes that affects the movement of ma- 

 terials into and through them. At the present 

 time it appears to be due to a number of quali- 

 ties; we shall mention only the simplest ones 

 here. A differentially permeable membrane 

 should not always be visualized as a crude sort 

 of sieve through which particles can or cannot 

 pass according to their size. This fact is illus- 

 trated by the differential permeability of rub- 

 ber membranes, through which the larger 

 molecules of carbon dioxide pass more readily 

 than the smaller molecules of water, oxygen, 

 and nitrogen. 



One of the features of permeability appears 

 to be illustrated by a simple demonstration in 

 which the membrane consists only of water 

 (Fig. 70). Ether diffuses through a water 

 membrane more rapidly than chloroform. Since 

 ether is much more soluble in water than 

 chloroform, this demonstration indicates that 

 solubility of the diffusing substance in the 

 membrane is one of the factors of permeabil- 

 ity. Solubility in turn seems to depend upon 

 the relative attraction between the molecules 

 of the substances involved. If the membrane 

 were equally permeable to these two substances, ether would still diffuse 

 through it the more rapidly because its diffusion rate is about 1.6 times 

 that of chloroform. 



The most interesting features about the permeability of protoplasm 

 are (1) its variability, and (2) the fact that the continuation of life 

 depends upon its variation remaining within certain limits. A great num- 

 ber of conditions and substances increase or decrease the permeability 



■EtKer 



_Water 

 'Ynembraae""' 



Chloroform 



B 



Fig. 70. Diagram of ap- 

 paratus used to demon- 

 strate the differential per- 

 meability of water to ether 

 and chloroform: A, when 

 first set up; B, several 

 days later. 



