WINTER INJURY 261 



tender. From his experiments he concludes that apple roots 

 will be killed at about —3° C. in summer when they are ten- 

 derest and at about -12° C. in late winter with rapid freez- 

 ing, this varying somewhat with conditions. He also shows 

 that French crab stock is less hardy in the roots than the 

 cion-roots of such varieties as Ben Davis; that "Marianna 

 plum roots are more hardy than Myrobolan roots, and IVIah- 

 aleb cheriy roots seem slightly more hardly than Mazzard 

 roots." 



231. How freezing kills. — A distinction is made between 

 the loss of fruit crops by ''killing frosts" and by "freezing." 

 The latter is here considered in studying the destruction of 

 the tissues of fruit-trees. It is generally accepted that 

 the freezing to death of plant tissues does not take place 

 unless ice crystals are formed within the plant from water 

 that has been withdrawn from the cells. While ice crystals 

 may form within the cells themselves when the temperature 

 falls veiy rapidly, the above method is much the more com- 

 mon. It was formerly considered that much less injury 

 w'ould result if thawing of the tissue was gradual so that the 

 cells could again take up the moisture, assuming, of course, 

 that the cells had not been ruptured in the process of freez- 

 ing. Later investigators ^ have, however, shown that the 

 rate of thawing has nothing to do with the killing of the tis- 

 sue, except with ripe apples and pears, lettuce, and the 

 leaves of Agave americana, but that the killing occurs when 

 a sufficiently low temperature is reached. 



"Frozen to death" is a technical phrase describing plant 

 tissues that have been subjected to a certain temperature 

 at which death of their cells occurs. Such tissues present a 

 brown, water-soaked appearance shortly after they thaw 

 and evaporation is much more rapid than from living tissue. 



' Miiller-Thurgau, Chandler, et al Mo. Agr. Exp. Sta. Res. Bull- S. 

 p. 150. 



