COLD AND GROWTH OF PLANTS — COVILLE. 289 
chilling required 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 dor- 
mant, 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 distribution 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 chill- 
ing 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 artifically the periodic 
chilling they require. 
When it became evident from the earlier observations and ex- 
periments that chilling played so essential a part in the behavior of 
our trees and shrubs it was clear that additional experiments ought 
to be conducted in which actively growing plants might be sub- 
jected to chilling temperatures without being put in a dark place 
like the ordinary refrigerator. To meet the requirement of both 
cold and light a glass-covered, outdoor, brick chamber was con- 
structed in 1912. It was kept above freezing by heating with electric 
lights, which were turned on and oft* automatically by a simple 
thermostat. In summer the chamber was kept cool, though not 
really cold, by means of ice and electric fans. Although much was 
learned with this apparatus it Avas crude and inadequate. To pro- 
Adde for more exact experiments a glass-covered compartment chilled 
by a refrigerating machine was constructed in one of the Department 
of Agriculture greenhouses. The refrigerating apparatus is a sul- 
phur-dioxide machine having a refrigerating power equivalent to 
1,000 pounds of ice a day. It is run by a 2-horsepower electric 
motor, and it furnishes ample refrigeration for the lighted com- 
partment, which is a glass-covered frame 25 feet long, 3 feet wide, 
and 14 to 20 inches in depth. The first of these refrigerated frames 
was devised and constructed in 1916. In this enterprise I had the 
valued advice and assistance of Dr. Lyman J. Briggs. The useful- 
