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freezing" in iiaturo could never be observed. 6. A mechan- 

 ical injury resulting in the destruction of the fine struc- 

 ture of protoplasm by ice crystals has been invoked as 

 the cause of death. 7. Some investigators have thought of 

 a damage by pressure between growing ice masses. 8. 

 Others have proposed that the protoplasm might be in- 

 jured by the jarring involved in freezing and thawing. 



9. A large number of investigators have studied exper- 

 imentally the theory that the so-called death by freez- 

 ing is caused by a too rapid thawing. Xo general con- 

 clusion can be derived from their results which seem 

 largely to disagree. 



10. Most plant physiologists have attributed death by 

 freezing to a dehydration of protoplasm. Some consider 

 dehydration as a step toward a precipitation or a coagu- 

 lation; others think of a destruction of the structure of 

 living molecules, perhaps of catalysts, by the withdrawal 

 of their constituent water; for others dehydration might 

 cause such essential changes as an increase in permea- 

 bility, an increase in viscosity, an upsetting of the ionic 

 balance, etc. ; finally, some authors attribute death to the 

 toxic concentration resulting from dehydration. 11. Ar- 

 guments in favor of the dehydration theory of freezing 

 are: the resemblance, repeatedly recognized, between the 

 action of freezing and that of drying and the resem- 

 blance in the conditions of sensitivity to these two fac- 

 tors. 12. The main objections against the theory are: 

 that in some plants more water can be removed without 

 injury by other means than by freezing ; that dehydration 

 by freezing is assumed to be lethal, while dehydration 

 by air-drying increases the resistance to cold. 



13. Various physiological changes which accompany 

 freezing have been assumed to cause injury : a weaken- 

 ing of vitality liy cold, an impairment of the synthesizing 

 ability, an impairment of functions such as regeneration, 

 etc. 1-4. Among the physical or chemical changes which 

 might be responsible for death by freezing the most im- 



