444 BOTANICAL GAZETTE [may 



It has been observed during spring frosts that in blooming apricots 

 those buds which have opened and are turned upward to collect snow 

 are frozen, while those turned downward may not be injured. West 

 and Edlefsen^ attempted to prevent frost injury to apricot buds by 

 spraying the trees with water. Instead of the desired effect of preventing 

 freezing, this procedure evidently killed the tissue by allowing inoculation 

 from ice formed on the surface. Trees which were not sprayed with 

 water were not injured, although subjected to the same temperature. 

 These examples serve to illustrate the importance of surface inoculation 

 in producing frost injury. 



The amount of undercooling in plants is not generally very great, 

 nor is it sufficient to account for true frost hardiness. Such herbaceous 

 plants as cabbage, kale, turnips, however, which ordinarily can with- 

 stand a considerable degree of freezing, acquire hardiness quite rapidly. 

 In the case of cabbage, a temperature of 3°C. was found to harden the 

 plants sufficiently in 5 days to allow them to be frozen stiff at — 3°C. 

 without injury. The principal importance, therefore, of the epidermal 

 coverings for the frost resistance of such plants appears to be that they 

 allow the plants which possess them to withstand temperatures somewhat 

 below zero until the cells are able to adapt themselves physiologically 

 to the changes incident upon freezing. 



Summary 



Undercooling of the tissues occurs to a greater degree in such her- 

 baceous plants as possess protective epidermal coverings than in plants 

 not so protected. The undercooHng in such plants is not due to sub- 

 stances in the cell sap, but mainly to the prevention of inoculation from 

 ice formed on the surface of the tissue. A method is given for determin- 

 ing electrically the temperatures within leaf tissues. — R. B. Harvey, 

 Bureau of Plant Industry, Department of Agriculture, Washington, D.C. 



3 West, F. S., and Edlefsen, N. E., Orchard heating. Bull. 161, Utah Agric. 

 Coll. Exp. Sta. 1917. 



