157 



ditioiis: 1. The liorizoiilal plateau was found higlier in 

 dead than in living tissues (for l)ibliographical references, 

 see above under "Freezing Points") ; 2. In dead tissues, 

 the plateau was maintained horizontal for a longer time and 

 then the curve dropped more rapidly than in living tissues 

 (Fig. 20) ; 3. The freezing curves of living material often 



5 10 15 20 MIN 



Fig. 20. Comparative freezing curves of living (A) and dead (B) plant 

 tissue. (From Luyet and Gehenio, 1937.) 



showed jerks or irregularities while those of the dead were 

 smoother ; 4. In dead material, the position of the horizon- 

 tal plateau, that is, of the freezing point, was little affected 

 by the cooling velocities used in the experiments, except 

 at the very high velocities of 5 degrees per minute ; in liv- 

 ing material, the horizontal part of the curve was lowered 

 by cooling velocities of some 3 degrees per minute. 



Several of the authors mentioned above have attempted 

 to explain these differences. In general, they attribute 

 most of the results to the resistance offered by the living 

 cell membranes and by the living protoplasm to the with- 

 drawal of water. The dead tissues, in which the mem- 

 branes have become permeable, and in which the proto- 

 plasmic structure has been altered, would behave as 

 aqueous solutions ; their water would be free. The living 

 tissues in which it is thought, water is held more firmly, 

 would release the latter only on the application of more 

 force. The jerks are attributed by Luyet and Gehenio to 

 the sudden release of various amounts of water or to the 



