122 



lions on j)ot;(to, ;i mci-cnrv 1 lici'nioinctcr was iiiscrlcd into 

 a cavity bored in the tuber; for those on leaves, the bulb of 

 the tlu'rmometer was wra|)i)ed in tliese organs. The cause 

 of the differences observed is attributed to the fact that 

 the eai)illai\\' spaces Ix'twcen the intact niembi-anes of tlie 

 cells in the living tissues hold water more tirmly and rcndci- 

 freezing more (,lif^icult than in dead material. 



■Maximov (1914-) obsei'ved. similar differences in red 

 beet; he obtained -2.15° and -1.25° for living and dead 

 material, respectively, and -1.21° for the extracted beet 

 juice. According to this author, the resistance presented 

 by the living cellular membranes to the outward passage 

 of water during freezing is responsible for the lower freez- 

 ing point of the living material. In dead tissue, the sap 

 forms a uniform fluid mass which freezes like a solution, 

 there being no membrane resistance to overcome ; in living 

 tissue, the sap has to be extruded from the cells before it 

 freezes ; it is extruded in small quantities at a time and the 

 heat developed by the freezing of such small quantities is 

 not enough to raise the temperature of the whole tissue to 

 its real freezing point. 



Walter and Weismann (1936) found, in pieces of potato 

 frozen several times in succession, a rise of the freezing 

 point at each of the first few congelations (for example, 

 -0.89°, -0.76°, -0.72°, -0.69°, in a series of 4 successive 

 experiments). Thereupon the freezing points oscillated 

 in an irregular manner. These authors determined by the 

 1)rowning of the tissue and the loss of turgor the increasing 

 num])er of dead cells in successive freezings and they claim 

 that it is the proportion of living cells which determines 

 the level of the freezing point. Dead matter freezes first 

 and the heat produced during its congelation, being ab- 

 sorbed by the bulk of surviving material, cannot I'aise the 

 temperature of the whole to the same level as when all the 

 cells are dead. Besides, in completely dead material, the 

 freezing point may be high because the sap is diluted by 

 the water which was bound in the living state and which is 

 set free at death. 



