THE INFLUENCE OF COLD IN STIMULATING THE 
GROWTH OF PLANTS. 
By FREDERICK V. COVILLE, 
Botanist; United States Department of Agriculture. 
[With 27 plates.] 
In regions having a cold winter like ours, with prolonged or re- 
peated freezing, the native trees and shrubs become dormant in 
autumn. According 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 previ- 
ously to a period of chilling; and, finally, a theory will be advanced 
to explain this paradoxical effect of cold in stimulating growth in- 
stead of retarding it. 
The subject will be presented in a series of numbered statements, 
each followed by supporting evidence. 
1. 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 green- 
house experiments, the writer came upon a strange phenomenon 
which was wholly unexpected and which threatened to interfere 
seriously with the successs 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 
1 Address delivered Apr. 27, 1920, before the National Academy of Sciences. Re- 
printed from the Journal of Agricultural Research, vol. 20, pp. 151 to 160, 1920, 
281 
