38 THE PLANT WORLD. 



occurs when the ice-formation has progressed to a certain extent, 

 and bears no relation whatever to the thawing out as was once 

 supposed. Death seems due to the actual withdrawal of water 

 to form ice, not to the cold. The ice formation dries out the 

 cells and the plant suffers therefore from drought conditions. 

 Every cell has its critical point, the withdrawal of water beyond 

 which will cause the death of the cell, whether by ordinary evapo- 

 ration or by other means. It may be supposed that the delicate 

 structure of the protoplasm necessary to constitute living matter 

 can no longer sustain itself when too many molecules of water 

 are removed from its support. In the great majority of plants 

 this point lies so high in the water content that it is passed very 

 soon after the inception of ice-formation, hence the death of so 

 many plants at this period. Others may be able to exist with 

 so little water that a very low temperature is necessary before a 

 sufficient quantity is abstracted to cause death. From some 

 plants enough water cannot be extracted by cold to kill them. 

 This explanation seems the most plausible one so far advanced to 

 account for death by freezing. 



In conclusion, the principal points of the paper may be sum- 

 marized as follows : 



The older idea that the ice forms within the cell, and thus causes 

 death by rupturing the wall, has been shown to be erroneous. 



Except in a few cases, ice forms invariably in the intercellular 

 spaces, unless the cooling is more rapid than usually occurs in 

 nature. 



The ice-masses produced in the spaces are often large lenticular 

 structures. These are composed of ice prisms side by side in two 

 layers. Superficial ice crusts, and those produced on damp soil, 

 are similar to those in the tissue, but composed of only one layer 

 of crystals. These crystals grow only by additions at the end 

 in contact with the tissue. 



The water of which the crystals is composed is almost pure. 



The cells of frozen tissue, except when strong and woody, are 

 always in a more or less collapsed condition. 



Much more ice is separated from the tissues to form ice at 

 temperatures just below where freezing begins than at lower tem- 



