COLD AND GROWTH OF PLANTS—COVILLE. 991 
have been able to do unless we had first worked out the principle 
of chilling, an understanding of which was essential to our work of 
breeding and propagation. 
In conclusion I wish to express the opinion that the chilling ot 
dormant trees and shrubs of temperate climates as a prerequisite to 
their resumption of normal growth in spring ought to be recognized 
in books on plant physiology as one of the normal processes in plant 
life. These works should contain chapters on chilling, just as they 
now contain chapters on other fundamental factors and principles 
relating to the life history of plants. And especially in books on 
plant physiology in relation to agriculture should the subject of 
chilling be dealt with in detail, for when in the pursuit of agriculture 
we take plants from one part of the world to another, or undertake 
to grow them out of season, or attempt to propagate them in quan- 
tity by grafting or by other processes unknown in nature, we are 
greatly handicapped and limited in our operations if we do not 
understand the principles of a process so widely existent in nature 
and so indispensable to a large proportion of the plants of temperate 
agriculture as the process of chilling. 
