Killing of Plant Tissue by Low Temperature 205 



ture will fall very rapidly. Since rapid temperature fall near the freez- 

 ing point seems to be more harmful than rapid temperature fall near the 

 killing temperature, it would seem certain that greater killing should 

 thus result. It does not seem impossible that "sun scald" of apple 

 trees may be explained in this way. 



This rapid fall of temperature may also be a feature to be con- 

 sidered in heating an orchard. Thus any one who has worked with 

 orchard heaters knows that if on a still night a few of the heaters 

 go out, the temperature will immediately fall to about that which 

 would prevail without the heaters. In this case it is possible that 

 the tissue may kill worse than if the heaters had not been there, 

 since the blossoms or fruit would freeze very rapidly. We can not 

 be certain of this, however, for results with rapid freezing of blossoms 

 at this station have not been uniform enough to be conclusive. 



Maturity and Hardiness. Probably the greatest factor in deter- 

 mining the amount of cold that can be withstood by trees and shrubs 

 that live through the winter is a condition of maturity. Emerson^ 

 has studied the question of maturity of fruit and other trees in Ne- 

 braska and has found that the varieties most hardy in wood are 

 those that mature early. Where growth can be checked early in the 

 season, as by a gross feeding cover crop like millet, the trees will 

 also withstand more cold. 



Selby^ made a study of the injury to fruit trees and ornamentals 

 by the severe freeze early in the winter of 1903-4, and attributes 

 the severe injury to the fact that the trees grew late in the fall on 

 account of a very wet period following a period of dry weather. 



Eustace^ reports a study of the effect of the same winter on 

 fruit trees and describes similar conditions. It seems in this case 

 also the great injury is due to the trees' having grown late in autumn. 



Winkler^ found that the resistance of native trees of Germany 

 is least in May, June, July, and August, and gradually increases 

 during September, October, November and December, and is great- 

 est in January, as measured by laboratory freezings. 



In summer the fruit buds, for example in August after they can 

 be easily detected, may be killed by a temperature of -9 to -10 C. 

 or somewhat lower on some years. At this time there is little dilTer- 

 ence between the hardiness of the buds, the wood, and even the foli- 

 age, though the foliage kills slightly the worst; while in winter fruit 



'Nebraska Akp. Kxp. Sta. Bui. 70. 1003. (nibl. No. 33): Nc-h. Arp. Exp. Sta. Iliil. 02. 190G. 

 (Blbl. No. 34); Net). Akp. Kxi). Ht;i. .\iil. ICpt. No. 10, lOOli, pp. 101-10. (Hlbl. No. 3.')). 

 ><)hIo Agr. Exp. Sta. Hul. 102. lod.s. dUbl. No. 101). 

 Now York ((ienova) Agr. Kxp. Sta. Hul. -'(.0. lW)r>. (IHbl. No. 38). 

 Jahrb. f. WIss. Hot. Vol. Oli. 1013, pp. 4(17-50(1. (Hlbl. No. 121). 



