74 Plant Physiology 



tion, transport, and final accumulation (often without 

 change) of certain substances in special organs. There 

 are a number of factors affecting such relations, but much 

 is yet unexplainable on a physical basis. 



47. Protoplasmic permeability. It is an obvious fact 

 that the plasma membrane is permeable to certain solutes, 

 else no growth could result. It is as clearly apparent that 

 this membrane is impenetrable to certain other solutes, 

 and this implies selective absorption. The fact of imper- 

 meability becomes evident from a simple observation upon 

 colored cell-sap, and especially so upon contemplation of 

 the phenomenon of plasmolysis in cells containing colored 

 sap. The colored cell-sap of a red beet or of a cell from 

 a stamen-hair of Tradescantia does not diffuse into the 

 surrounding water so long as the cell is uninjured. More- 

 over, when such cells are plasmolyzed, there is, with con- 

 tinued health, no noticeable exosmosis of the colored 

 material. From dead cells there is prompt diffusion of 

 the colored sap. In this connection it is also to be re- 

 membered that in many cases red or blue color in plant 

 cells is merely an indication of acid or basic substances, 

 and this color may be changed in the living cell if it is per- 

 meable respectively to basic or acid compounds. 



Pfeffer has clearly demonstrated important facts regard- 

 ing permeability through his experiments upon the pene- 

 tration of dye stuffs. Methylene blue at - a strength of 

 1 part to 100,000 of water yields a solution which is not 

 visibly blue unless observed in a layer several centimeters 

 thick. It would not therefore give an evident coloration 

 in a plant cell. It is found, however, that upon being 

 placed in such a solution certain root-hairs, Spirogyra, 



