273 



iiu'iit of the eoiistitiu'iit parts of pi'otoi)lasiii, including 

 water, and that sneh destruction has been attributed to 

 one of the three following factors : temperature alone, 

 the withdrawal of water during freezing, some processes 

 which take place during thawing (three theories which, 

 according to him, can be traced liack to the previous 

 century.) After discussing the pros and cons for the 

 first and the third possibilities he gives his preference to 

 the second, saying that, since water withdrawal is the 

 most essential change which occurs during freezing, it 

 could very well be the cause of death by cold. 



He then goes a stejj further and gives a body to the 

 dehydration theory by suggesting a mechanism by which 

 death might result from the withdrawal of water. He 

 considers that the solid constituents of protoplasm, /. e., 

 the micelles, are separated by the water phase, being far- 

 ther apart in i)rotoplasni which has a higher water con- 

 tent : the withdrawal of water will result in displacing 

 these micelles from the positions that they occupied in 

 the ordered arrangement of living protoplasm. How a 

 molecular structure can be altered by freezing is exem- 

 plified by starch paste which, after congelation, loses its 

 property of holding water of imbibition. 



Miiller-Thurgau also holds that : 1. The sudden with- 

 drawal of water during the rapid freezing which follows 

 subcooling is particularly dangerous; 2. Often the dis- 

 turbance produced by dehydration would .be reversible 

 and the original structure could be restored if the water 

 witlldra^^^l by freezing would not evaporate after thawing 

 before it can be returned to the cells. Conseijuently, he 

 claims that a means of keeping alive frozen material is 

 to prevent evaporation during and after thawing. 



Several observed facts are interpreted by Miiller- 

 Thurgau as being in good fitting with his dehydration 

 theory. The greater sensitivity to cold in plants with 

 higher moisture content, as exhibited, for example, by 

 soaked seeds, and the relativelv higher resistance of less 



