Oct. 14, 1918 



Hardening Process in Plants 



99 



CHANGES OF REACTION ON PLASMOLYSIS 



A change in reaction can be produced also by placing sections of coleus 

 leaves in concentrated sugar solutions. The protoplasts of the trichomes 

 are plasmolyzed, and the red color is deepened by the removal of water. 

 After a time the color changes to blue, and at the same time the pigment 

 begins to lake from the cell, indicating a change of permeability and the 

 death of the tissue. If the plasmolysis is not carried too far nor allowed 

 to continue until a change of color occurs, the cells regain their turgidity, 

 and the pigment does not leach out. Hence, the duration of the reaction 

 appears to be a factor in producing the injury, as well as the amount of 

 change of acidity. 



Fig. I. — Graph showing change of the hydrogen-ion concentration of cabbage-leaf juice on freezing. 

 Freezing at point x, thawing at point y. 



POTENTIOMETRIC DETERMINATION OF THE CHANGES IN HYDROGEN- 

 ION CONCENTRATION ON FREEZING 



The change in acidity of a plant juice during freezing can be followed 

 by the use of potentiometric methods. The potentiometer arrangement 

 of Michaelis (54) was used. The measurements were made in a closed 

 vessel provided with a dip hydrogen electrode essentially like that of 

 Bovie (3) except that an exit tube was provided for the hydrogen. 



As shown by the graph (fig. i), the acidity of cabbage juice is increased 

 by freezing. There is first a rise in the hydrogen-ion concentration, then 

 a return to its original value, and then a very rapid rise to an acidity 



