262 FUNDAMENTALS OF FRUIT PRODUCTION 



this view, it having been found to hold only in a very few cases. As 

 already indicated, it can be shown definitely, sometimes at least, that 

 death occurs before any thawing begins; furthermore, the method used 

 by Sachs and much in vogue among gardeners to induce slow thawing, 

 namely, immersion or sprinkling with water, is in reality a method leading 

 to more rapid thawing than would occur in air. The water causes a coat- 

 ing of ice on the exterior, thus liberating heat to the tissue. Though ob- 

 jections might be adduced, to this view, the chief matter of interest here 

 is that investigators have found, in practically all cases, no difference 

 in killing to ensue whether the thawing be rapid or retarded. It should 

 be pointed out, however, that there are no reports of inquiry into the 

 effect of sunlight on frozen tissue. Injuries due to the effect of light 

 on frozen tissue might easily be attributed to rapid thawing. There 

 seems to be enough evidence in field conditions of association of sunlight 

 and injury to warrant careful study, particularly in view of the increased 

 permeability known to accompany increased light. 



VARIATION IN CRITICAL TEMPERATURES 



Definite evidence, under experimental conditions, has shown that 

 the critical temperature at which killing results is not a definite point 

 for any species, variety or individual plant but is the result of a complex 

 of conditions. It probably depends to a great degree on water-retaining 

 capacity or the amount of water present that is not readily frozen, but 

 other factors may be equally important under certain circumstances 

 and unquestionably there are a number of factors affecting the water- 

 retaining capacity of the cell colloids. All of these may constantly be 

 fluctuating more or less independently of one another and their product, 

 the killing temperature, must therefore assume many different values. 

 This is abundantly borne out in field observations of winter-killing. 



Summary. — The most tenable of the theories explaining killing from 

 cold ascribes death to dehydration of the cells. Ice formation generally 

 begins in the intercellular spaces and the process draws water from the 

 cells. The water is withdrawn gradually, each decrease in temperature 

 being followed by further water loss from the cells, though the rate of 

 this loss becomes progressively lower. Death occurs when the dehydra- 

 tion proceeds beyond a certain point. The increasing density of the 

 cell sap with continued water loss tends to hold the remaining water more 

 tenaciously and thus protects the cell somewhat against further loss 

 and eventual death. The cell colloids, particularly the water soluble 

 pentosans, operate in the same direction and play a still more important 

 part. These substances (the water soluble pentosans) develop in some 

 plants in response to certain environmental conditions — particularly 

 decreasing temperature and a decreased moisture supply. These facts 



