PHTSIOLOGT 45 



Although undissociated molecules appear to 

 enter much more rapidly than ions it is not to be 

 expected that all undissociated molecules enter 

 the protoplasm with the same readiness and in 

 many cases it appears as though they cannot 

 penetrate into the sap unless they can combine 

 chemically with some constituent of the proto- 

 plasm. 



Some such considerations may possibly explain 

 the excluding mechanism whereby, for example, 

 Valonia macro fhysa prevents more or less com- 

 pletely the entrance of undissociated molecules 

 of such substances as phenol, glucose, lactose, 

 sucrose, and many others, while admitting more 

 or less freely undissociated molecules of hydro- 

 gen sulphide, carbon dioxide, ammonia, ethyl 

 alcohol, dimethyl-glyoxime, paraphenylene- 

 diamine, and urea. 



It would therefore seem as if the surface 

 forms a barrier which more or less completely 

 prevents the passage of ions, and as if undis- 

 sociated molecules can enter the sap only by 

 passing through a layer which excludes such sub- 



