OSMOTIC PRESSURE IN RELATION TO PLANT DISTRIBUTION 175 



Their investigations were carried out by the method of 

 plasmolysis, as was also the more recent work of Falck 

 (1913) on the " alfvar " vegetation of the island of Oeland. 

 This type of plant association consists of stunted and 

 xerophytic species, and is similar to that found on the 

 steppes. Falck considers that the average normal value 

 for the osmotic pressure of plants is between 11 and 14 

 atmospheres, since they are isotonic with 0-3 to 0-35 gramme- 

 molecular potassium nitrate. There are, however, but 

 few of the alfvar plants which do not exceed this value 

 considerably. The highest value found by him was in 

 epidermal cells of Helianthemum oelandicum and amounted 

 to over 55 atmospheres, since 1-6 gramme-molecular potas- 

 sium nitrate was isotonic with them. 



References to many of the recent researches in this line 

 may be found in Falck 's paper, but it has been impossible 

 to mention more than a few of these here. 



A number of interesting examples of plants accommo- 

 dating themselves to an increase in the salinity of their 

 surroundings were studied by Cavara (1905). In the salt 

 lagoons of Cagliari evaporation proceeds vigorously from 

 the spring till early autumn. This leads to a corresponding 

 rise in osmotic pressure, as shown by Table XLIII. 



It is very probable that in these plants the increase in 

 pressure is due to the slow penetration of the salts of the 

 soil-water. Electrical conductivity measurements would 

 have decided this question, but they were not carried out. 



This power of adaptation, it may be added, is shown to 

 a very marked degree by the cells of Bacteria and Cyano- 

 phycese, and in these groups the protoplasm appears to be 

 extremely permeable. Raciborski (1905) succeeded in ob 

 taining the germination of Torula sp. in a saturated solu- 

 tion of lithium chloride. This possesses the highest osmotic 

 pressure of any known solution, namely, 965-3 atmospheres, 



