60 VEGETABLE PHYSIOLOGY 



the vacuole showed that the internal one prevented the 

 osmotic substances, originally present in the water which 

 the cell contained, from escaping in the issuing osmotic 

 stream. These substances must have been left behind, or 

 there would have been no osmotically active material to 

 draw the water back, when it was allowed to replace the 

 salt solution outside the cell. 



That this behaviour is dependent on the vital activity 

 of the protoplasm can be shown by repeating the experi- 

 ment after killing the living substance by a short immer- 

 sion of the cell in alcohol. Then the process of osmosis 

 goes on exactly as in the first experiment described. The 

 salt solution penetrates into the vacuole as if only a cellulose 

 septum were present, the dead protoplasm exerting no regu- 

 lating influence. 



We must not conclude from this experiment that inor- 

 ganic salts in all degrees of concentration are kept from 

 entering the cell by the protoplasm. If extremely dilute 

 solutions are employed, the living substance permits their 

 passage together with a certain appropriate amount of 

 water. Similarly, extremely dilute solutions of bodies 

 found in the fluid of the vacuoles, the so-called cell-sap, 

 can make their way out of the cells. The protoplasm 

 exerts a definite regulating influence, however, upon both 

 the entry and the escape of these different substances. 



The modified osmosis which is thus the mode of entry 

 of water into a cell containing no vacuole, and which 

 causes the growth or completion of the vacuole, after its 

 first appearance, continues after its formation is finished. 

 This can be studied most favourably in aggregations of 

 cells, such as we find in the cortex of a stem or the loose 

 mesophyll of a leaf, as in such cells there is a more evident 

 renewal of the water of the vacuoles than in those of 

 tissues which are surrounded by liquid. 



In such tissues as those just mentioned we can demon- 

 strate with ease what is more difficult to detect in the 

 others, that not only is water admitted to the cells, but it 



