1 58 Journal of Agricultural Research vol. xx, no. 2 



months of chilling are necessary before a newly dormant plant will 

 respond to the usual effect of warmth, such plants are protected against 

 the dangers of growth in Indian summer. It is probable that all our 

 native trees and shrubs are thus protected. 



Any member of this audience may make a simple and instructive 

 experiment next fall and winter with such early spring blooming plants 

 as alder, hazelnut, pussy willow, yellow bush jasmine, forsythia, Jap- 

 anese quince, peach, and plum. In mid-autumn bring into your living 

 room and set in water freshly cut, dormant, leafless branches of these 

 plants. They will not bloom. At intervals of a few weeks during late 

 autumn and winter try the same experiment again. You will find that 

 the branches cut at later dates will come into bloom under this 

 treatment. They will not do so, however, until the expiration of the 

 period of chilling appropriate to the various kinds of plants included in 

 the experiment. The required period of chilling varies greatly. For 

 some of the cultivated shrubs about Washington, especially the yellow 

 bush jasmine (Jasminum nudiflorum), so brief a period of chilling is 

 required that an extraordinarily cold period in late October or early 

 November may chill them sufficiently to induce them to bloom if a period 

 of warm weather follows in late November. The period of chilling re- 

 quired for the peach is so short that in Georgia unusually warm weather 

 in December sometimes brings the trees into flower, and their crop of 

 fruit is destroyed by the freezes that follow. 



From these facts it appears that our native trees and shrubs are so 

 intimately adjusted to the changes of the climate to which they have 

 been long subjected that they are almost completely protected from 

 injury by freezing, but some of the cultivated species brought from parts 

 of the world having a climate different from ours are only imperfectly 

 adapted to our climatic changes. They grow at times when our native 

 species have learned to hold themselves dormant, and they often suffer 

 severely in consequence. 



Chilling, as a protective adaptation, has become a physiological 

 necessity in the life history of cold-winter trees and shrubs. So fixed 

 indeed, is the habit that it appears to be a critical factor in determining 

 how far such plants may go in the extension of their geographic distri- 

 bution toward the Tropics. In the Tropics our common northern fruit 

 trees, apples, pears, peaches, cherries, grow well for a time and then 

 become half dormant. In the absence of chilling they never fully recover 

 from their dormancy; they grow with weakened vitality and finally die. 

 If these fruits are to be grown successfully in the Tropics they must be 

 given artificially the periodic chilling they require. 



When it became evident from the earlier observations and experi- 

 ments that chilling played so essential a part in the behavior of our trees 

 and shrubs, it was clear that additional experiments ought to be con- 

 ducted in which actively growing plants might be subjected to chilling 



