53 



out of 14 evergreens were not killed at - 19°. 4. The 

 maximum resistance is reached in Jamiary, at which time 

 the buds of all the evergreens (14 species) and of 27 out 

 of 29 species of deciduous trees were not killed at - 20° ; 

 most of them survived - 22° as well. In none of the spe- 

 cies were the cambium, cortex and wood injured at this 

 time by -20° or -22°. 5. Branches, whose death point 

 had been determined as about -21°, were frozen repeat- 

 edly at - 13°, and, after each freezing, they were kept for 

 24 hours under a bell jar at 18°. Most of the trees were 

 killed by a 6-times repeated freezing at - 13° ; the leaves 

 and needles of the evergreens were, in general, found more 

 resistant. 



While, as we have said above, the cells of juicy leaves 

 are generally thought to be killed when they are subjected 

 to temperatures of a few degrees below zero, Iljin (1933) 

 showed that sections of red cabbage leaves could be main- 

 tained alive after freezing at much lower temperatures if 

 the withdrawal and the reabsorption of water, concomi- 

 tant respectively Avith freezing and thawing, were gradual. 

 According to him, death results usually from a tearing of 

 the protoplast in a too rapid plasmolysis during freezing 

 or mostly in a too rapid invasion of the cell by water when 

 the ice melts. Freezing slowly and thawing in hypertonic 

 solutions would prevent such injury and maintain the 

 cells alive. So, he cooled the material by steps, letting it, 

 for example, one day at about -5°, one day at -8°, one 

 day at -11°, etc.; then he warmed it to about -5° and 

 immersed it in cooled salt solutions which were approxi- 

 mately isotonic with the frozen cell content ; he then let 

 the tissue thaw slowly in that solution. Proceeding in 

 this manner, he obtained living cells (capable of plasmoly- 

 sis) after an exposure of 21 hours to - 80° and after longer 

 exposure to the higher temperatures of - 15.8°, - 11.3°, 

 etc. If the observations of Iljin, which favor the old 

 theory that death is due primarily to a too rapid thawing 

 and not to congelation or to an injury at some given low 

 temperature, are confirmed, the classical notion of death 



