SELECTIVE PERMEABILITY 75 



as semipermeable in the absence of any 

 such definite structure. 



The fact that the surface excludes cer- 

 tain substances is very important. This 

 may be illustrated by the case of certain 

 fungi which grow on fairly concentrated 

 solutions of copper sulphate. Since cop- 

 per is poisonous this seems rather re- 

 markable. Analysis shows no copper in 

 the cells of the plant and it therefore 

 seems unable to penetrate. 



It is quite possible that hay fever fur- 

 nishes another illustration. Although the 

 facts are not by any means perfectly clear 

 it seems probable that if the proteins 

 of various kinds of pollen are absorbed 

 by the body the system becomes sen- 

 sitized to that kind of protein. The case 

 is quite the opposite of immunity where 

 the introduction of a poison renders the 

 body less sensitive. It is well known that 

 each species of animal has its own char- 

 acteristic protein and that all other pro- 

 teins act as poisons. Ordinarily when 

 pollen enters the nose or mouth its pro- 



