126 TEXTBOOK OF PLANT PHYSIOLOGY 



Not all plants succumb with equal ease to the formation of 

 ice. Some of them, the most sensitive ones, are killed in the very- 

 beginning of this process. Others, such as the cabbage or the 

 onion, are able to stand moderate freezing. Those in a third group 

 may freeze to a quite hard condition and thaw out afterwards 

 without losing their vitality. Many plants of the polar regions 

 have this capacity. Some winter cereals are able to stand 15 to 

 20° C. even in snowless winters. Still more cold resistant are the 

 dormant buds of deciduous trees and the needles of conifers. 



The reason for this hardiness was sought first in the fact that 

 because of the small amount of water in these dormant organs, 

 especially the buds and needles, no ice could form. This suppo- 

 sition, however, has proved erroneous. Observations have shown 

 that even these organs freeze at 15 to 20° C. With still lower 

 temperatures they become brittle as glass. Moreover, all such 

 external adaptations as the layer of cork covering the branches, 

 the bud scales, etc., which, for a long time, were regarded as protec- 

 tions against cold, in reality are unable to check heat loss for any 

 considerable length of time. The role of these adaptations is a 

 different one, namely, the protection against desiccation during the 

 winter, which threatens the aerial parts of the plant as a result of 

 the decreased water supply from the frozen soil. Frost resistance 

 is not a capacity of dormant plants to protect themselves against 

 the formation of ice, but a property of being very little or not at 

 all affected by its formation. 



This lack of susceptibility is attained due to chemical changes 

 in the cells. It has been noticed for a long time that in the cold 

 season leaves of plants, though wintering under snow, show no 

 traces of starch. Considerable amounts of sugar are accumulated 

 instead. Experiments have shown that sugar has the property of 

 preventing the coagulation of albuminous substances by low tem- 

 peratures. It therefore may be called a protective substance. 



This protective influence of sugar may be readily demon- 

 strated by means of the following simple experiment: Take three 

 test-tubes containing the sap squeezed from a plant. This sap 

 always contains a certain amount of protein. One of these tubes 

 should be left as a check, the two others should be frozen, after a 

 little sugar has been added to one of them. After thawing of the 

 contents of both tubes, it is seen that in the tube which received 

 sugar the proteins are in the same condition, a sol, as in the control. 



