76 Plant Physiology 



with respect to permeability, and there has seemed to be 

 some relation between the capacity to penetrate and the 

 solubility of the solute in cholesterin or other similar com- 

 pound. This has naturally led to the assumption that 

 some such substance constitutes an important part of the 

 plasma membrane. Nevertheless, there are many ex- 

 ceptional cases, and different plants frequently exhibit 

 marked specific peculiarities. 



The differences in penetration referred to are charac- 

 teristic also of toxic or injurious compounds as well as of 

 nutrient or beneficial substances. This fact is frequently 

 of service in explaining the relative toxicity of different 

 reagents. Nowhere is this shown more clearlv than in the 



^ 



experiments of Brown, from which it is evident that the 

 seeds of barley may be placed for a considerable time in a 

 relatively strong solution of sulfuric acid without injury, 

 whereas mercuric bichloride rapidly effects an entrance 

 and kills the cells. Further details of this experiment 

 are cited later (section 263). The illuminating experi- 

 ments of Kahlenberg on osmosis demonstrate clearly that 

 the nature of the semipermeable membrane is a matter 

 of great importance in osmotic phenomena. Further- 

 more, it has been shown that external conditions, includ- 

 ing those of temperature, light, and nutrition, affect 

 permeability and selective absorption to a high degree. 

 The plasma membrane should be regarded as made up 

 in part of a variable and complex colloidal solution. 



48. The role of diffusion and osmotic pressure. - - From 

 what has been brought forward respecting osmosis and 

 diffusion it can be said that these forces are conspicuous 

 in the work of the cell. The concentration of the cell- 



