THE INFLUENCE OF COLD IN STIMULATING THE 

 GROWTH OF PLANTS * 



By Frederick V. CovrtLE 



Botanist in Charge, Office of Economic and Systematic Botany, Bureau of Plant 



Industry, United States Department of Agriculture 



In regions having a cold winter like ours, with prolonged or repeated 

 freezing, the native trees and shrubs become dormant in autumn. Ac- 

 cording to the general belief this condition is brought about by the cold. 

 It is also the general belief that warm weather is of itself the sufficient 

 cause of the beginning of new growth in spring. Both these ideas are 

 erroneous. It is the object of the present address to show : first, that 

 in our native trees and shrubs dormancy sets in before cold weather, and 

 that cold weather is not necessary for the establishment of complete 

 dormancy; second, that after such dormancy has begun, the exposure of 

 the plants to an ordinary growing temperature does not suffice to start 

 them into growth; third, that these plants will not resume normal growth 

 in the warm weather of spring unless they have been subjected pre- 

 viously to a period of chilling; and, finally, a theory will be advanced 

 to explain this paradoxical effect of cold in stimulating growth instead of 

 retarding it. 



The subject will be presented in a series of numbered statements, each 

 followed by supporting evidence. 



i. Trees AND SHRUBS of cold climates become dormant at the 



END OF THE GROWING SEASON WITHOUT THE NECESSITY OF EXPOSURE 

 TO COLD WEATHER. 



A little more than 10 years ago, while engaged in a series of greenhouse 

 experiments, the speaker came upon a strange phenomenon which was 

 wholly unexpected and which threatened to interfere seriously with the 

 success of the experiments. Healthy blueberry plants, intended to be 

 used during the winter for breeding purposes, were brought into the 

 greenhouse at the end of summer and were kept at an ordinary growing 

 temperature. They refused to continue their growth during the autumn, 

 gradually dropped their leaves, and went into a condition of complete 

 dormancy. They did this at a greenhouse temperature which in spring 

 and summer would have kept the plants in a condition of luxuriant 

 growth. The completeness of the condition of dormancy which such 

 plants reach can be best appreciated from photographs (PI. 20, A). 



Since 1910 this experiment has been repeated many times and with 

 many species of plants, and without exception those trees and shrubs 



1 An address delivered before the National Academy of Sciences Apr. 27, 1920. 



Journal of Agricultural Research, Vol. XX, No. 2 



Washington, D. C. Oct. 15, 1920 



vg Key No. G-205 



(151) 



