V.)'2 



Wright and Taylor (1921) remarked thai water or sap 

 from ])riiises on the surfaee of a ])()tato ])i'eveiited that 

 material from reaching' the degree of undercooling that it 

 would attain without tiie presence of such moisture. 



On the other hand, Luyet and Hodapp (1938b) repeat- 

 edly obtained subcooling, and often to a considerable de- 

 gree, when cylindrical i)ieces of potato tissue were exposed 

 to low temperatures in glass vials previously filled with 

 water. Moisture, in this case, did not facilitate freezing. 

 One must notice, however, that this moisture might have 

 lost its ordinary properties due to the fact that it was held, 

 in the form of a capillary layer, between the material and 

 the vial. 



Concerning the efficacy of seeding by surface moisture, 

 it seems that if such moisture can be brought to a lower 

 temperature than the tissue itself (as, for example, by 

 contact with a cooler object), or if it has a higher freezing- 

 point than cell sap, inoculation becomes understandable; 

 but one cannot see why a film of moisture of capillary size, 

 mixed with sap at the surface of a cut tissue, would not 

 subcool as much as the tissue itself. 



Luyet and Gibbs (1937) remarked that seeding through 

 cellular membranes must be impossible in living tissues 

 like onion epidermis, in which one can observe the sub- 

 cooled cells freeze one by one, there being sometimes half- 

 hour intervals between the congelation of neighbouring- 

 cells, as was recorded previously by Molisch (1897). 



3. The Time Factor. Some authors claim that, if a 

 liquid is maintained long enough in the subcooled state, 

 the molecules will unfailingly have a chance of colliding 

 in the conditions of orientation necessary for crystalliza- 

 tion. This view is based on thermodynamic considera- 

 tions, but what is somewhat puzzling is that the time 

 required for the formation of nuclei at low and at high 

 subcooling temperatures does not seem to vary in proj:»or- 

 tion to the change in thermodynamic activity of the mole- 

 cules. While at low temperatures one can maintain the 

 subcooled state for but a few seconds, at slightly higher 

 temperatures one can sometimes maintain it for years. 



