260 



FUNDAMENTALS OF FRUIT PRODUCTION 



Table 17. — Pentosan Content of Garden Plants in Percentages op Fresh 



Weight'"'' 



Increased Hardiness with Increased Maturity.^ — The most generally 

 recognized and most potent single factor influencing killing by cold, 

 particularly in tissues withstanding a fair amount of freezing, is the 

 degree of maturity attained at the time of exposure. So widely is this 

 state recognized in field conditions, that experimental evidence on this 

 point, though available, is hardly necessary. Some less known, but 

 widely occurring, phases of immaturity in trees are considered later. 

 The greater susceptibility of immature tissue to injury from cold is due, 

 in part, to the fact that pentosans or other water-retaining substances 

 have not developed : The greater water content of such tissue is evidence 

 of the lack of those drying conditions necessary for the proper develop- 

 ment of pentosans and hence of maturity. Chandler found no constant 

 difference in the moisture content of the twig cortex during the winter, 

 though its hardiness varied considerably. With reference to density he 

 states: "It would seem certain then that while a part of the increased 

 hardiness of tree tissue in winter may possibly be accounted for by the 

 greater sap density, not all of it can ; certainly not the greater hardiness of 

 December tissue over that of October." The same investigator offers the 

 following suggestion based on experimental evidence; "It would seem 

 highly probable that, except in the case of cambium, the additional hardi- 

 ness acquired by the different tissues of the trees as they pass into winter, 

 is a change in the protoplasm such that it can withstand the great loss 

 of water rather than a change in the percentage of moisture or in sap 

 density." 



RAPID TEMPERATURE CHANGES 



Since maturity is a reaction to dry conditions whether produced by 

 exposure to cold or by actual limitation of the water supply, it is logical 

 to expect distinct differences in the amount of injury produced by rapid 

 freezing when no time is allowed for the development of pentosans and 

 by a gradual reduction of temperature permitting an increase in the 

 water-retaining capacity of the tissue to develop. 



Killing with Slow and with Rapid Freezing. — The injurious effects 

 of rapid freezing seem to have received little attention until the recent 

 work of Winkler and of Chandler, 



