Oct. 14, 1918 Hardening Process in Plants 107 



gen-ion concentration is decreased on thawing. This is probably the 

 case in the frozen cells of the injected areas of nonhardened cabbages. 

 In those cells which survive one would then expect to find a slightly 

 decreased acidity. No indicator covering the proper range of acidity 

 has been found which will penetrate the cabbage cells in sufficient con- 

 centration to show a color deep enough not to be masked by the 

 chlorophyll. However, such a change is indicated by the change of the 

 injected spots on coleus leaves from red to blue. The effect of the 

 decreased acidity may be to cause greater activity of the peroxidase 

 present or to allow its accumulation. Blackening occurs in the areas 

 of Aucuha japonica injected by freezing, indicating an increased activity 

 of the oxidizing enzyms. Krasnosselsky {22) reports an increase in the 

 concentration of the oxidizing enzyms due to wound stimulus. It is 

 known that the oxidizing enzymes are destroyed quite rapidly at the 

 hydrogen-ion concentration shown by cabbage leaves and that a decrease 

 in acidity favors their action. 



SUMMARY 



(i) The first indications of frost injury to succulent plants were 

 observed in the appearance of injected areas over the leaf surface. 

 These injected areas are caused by the withdrawal of water from the 

 protoplast and the displacement of air in the intercellular spaces. Inocu- 

 lation of the undercooled leaf tissue from ice formed on the surface is 

 generally the cause of the local freezing. Wax on the leaf surface pre- 

 vents the inoculation of the undercooled tissue and thus prevents injury 

 from freezing. 



(2) Frozen cells in the leaves of cabbage, bryophyllum, salvia, and 

 lettuce were found to be stimulated to growth and to produce tumors 

 similar to those shown in pathological conditions, but without the presence 

 of bacteria. Frozen spots on the leaves of tomato, coleus, geranium, and 

 a number of other plants did not receive a growth stimulus as in the 

 former cases, but were killed by the freezing. This local kilHng of the 

 tissue gives such leaves a spotted appearance. 



(3) The peroxidase content of the intumescences of cabbage induced 

 by freezing was found to be much greatei than that for normal leaf 

 tissue. A decrease in the hydrogen-ion concentration may occur in such 

 cells, and this condition may allow greater activity or accumulation of 

 the respiratory enzyms, especially peroxidase. 



(4) It is suggested that the growth stimulus in frozen cells is not due 

 to the removal of the correlation effect on plasmolysis of the cells, but 

 is to be ascribed to a partial precipitation of the proteins of these cells. 

 This precipitation results in an increase in the permeabiUty of the cells 

 to water, and in the ability of the cells to hold sugars. 



(5) The apparent osmotic pressure on plasmolysis of the tumor cells 

 was found to be less than that for normal mesophyll cells. However, the 

 tumored areas of the leaf do not freeze more readily than the other areas. 



