92 Journal of Agricultural Research voi. xv, No. 2 



permeability alone do not effect the accumulation of substances within 

 cells. 



So far the writer has not obtained the freezing point of juice expressed 

 from cabbage tumors caused by frost injury. However, some evidence 

 of the relative freezing points of the tumor cells and leaf cells can be 

 gained from the following experiment: Cabbage plants showing an 

 abundance of young tumors were placed at a constant temperature just 

 sufficiently low to freeze the tissue. It was observed that on freezing 

 the leaves were frozen in spots over the areas not covered by tumors 

 usually before the tumor cells were frozen. One would expect the 

 tumors to freeze much more quickly than the normal tissue if the relative 

 osmotic concentrations were those shown by-plasmolysis. The failure of 

 the tumors to freeze can not be attributed to greater undercooling in 

 them, for the cells have thin walls and are not covered by wax. 



PHYSIOLOGICAL CHANGE IN CABBAGES DURING THE HARDENING 



PROCESS 



The physical changes in cabbage which have been observed during the 

 hardening process are a slowing of the growth rate so that the plants are 

 smaller than those of the same age grown at a warm temperature and 

 consequently are more mature; and an increase in the amount of bloom 

 on the leaves. The condition of hardiness can well be judged by the stiff, 

 springy condition of the leaves. Hardened leaves are 20 per cent thicker 

 than nonhardened leaves of the same age. 



By exposure to temperatures a few degrees above the freezing point 

 for a week or so the Early Jersey Wakefield cabbage acquires the ability 

 to be frozen stiff without injury, or with only slight injury (PI. 11, A). 

 Plants frozen directly after being taken from the greenhouse are either 

 injured in spots or killed throughout, according to the extent of the frozen 

 area and the temperature. Tomatoes under similar hardening treat- 

 meat can be made to withstand somewhat lower temperatures without 

 freezing; but once the hardened plants becomje frozen they are killed 

 (PI. II, B). Tomatoes are killed or injured on long exposure to a 

 temperature of 5° C. This injuring is not a true freezing to death, 

 according to Miiller {37, 39), for no ice formation takes place in the 

 tissue. Molisch {36) explains it on the basis of accumulation of toxic 

 substances through poor oxidation. Tomatoes which can not survive 

 ice forma-tion within the tissue can hardly be said to show a true hardiness 

 to frost. The teinn "hardiness" should be applied to the ability of a 

 plant to survive ice formation within its tissues, as shown by cabbage. 



A condition of greater resistance to freezing can be produced in cab- 

 bages by watering them with solutions which check growth. Plants 

 watered with nutrient solutions which produce rapid growth, such as 

 N}io potassium nitrate, Nlio calcium nitrate, or Knop's 2 per cent, are 



