60 VEGETABLE PHYSIOLOGY 



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 en- 

 tering the cell by the protoplasmic membranes. If extremely 

 dilute solutions are employed, the protoplasm 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 demonstrate with ease what 

 is more difficult to detect in the others, that not only is 

 water admitted to the cells, but it is also given off from 

 them. This does not depend on osmosis in the stem or 

 leaf, but is due to evaporation, which takes place from the 

 surfaces of the cells abutting on the intercellular spaces, 

 whence the watery vapour is exhaled through the stomata, 

 or, in the case of a woody stem, through the lenticels. In a 

 cell surrounded by water such removal must depend upon 

 osmotic currents. 



