434 
BOTANY: F. V. COVILLE 
Proc. N. a. S. 
THE INFLUENCE OF COLD IN STIMULATING THE GROWTH 
OF PLANTS 
By Frederick V. CovillE 
Smithsonian Institution, U. S. National Museum, Washington, D. C. 
Communicated by C. D. Walcott. Read before the Academy, April 27, 1920 
In regions having a cold winter like ours, with prolonged or repeated 
freezing, the native trees and shrubs, according to the general belief, 
become dormant because of 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. The evidence now 
presented shows: 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 start them into growth; third, that these plants will not resume 
normal growth in the warm weather of spring unless they have been sub- 
jected previously to a period of chilling. 
Dormant trees and shrubs which have had two or three months of 
chilling, either out of doors or in artificial cold storage, start into growth 
in the normal manner when spring comes, but if the dormant plants have 
been kept warm all winter they do not start into growth at the usual 
time in spring but continue their dormant condition for weeks or months, 
sometimes for a whole year, and when finally they do begin to grow their 
growth is of an abnormal character. One of these long dormant plants 
can readily be started into healthy growth, however, even after the ex- 
piration of a year, by subjecting it to a period of chilling. The best 
temperature for chilling is 32° to 40° Fahrenheit. It may be applied in 
either light or darkness. 
In one of the greenhouses of the Department of Agriculture refrigerating 
machines have been installed in such a manner that plants supplied with 
normal daylight can be subjected at any period of the year to winter 
temperatures, even below freezing if desired. 
By subjecting one part of a dormant bush to a chilling temperature and 
keeping the other part of it warm, the chilled portion can be brought into 
full leaf and flower while the other part remains completely dormant. 
An important change takes place in the plant during the process of 
chilling. The starch stored in the cells is transformed to sugar, and not 
until this has been done can the plant utilize its store of starch in making 
its spring growth. Furthermore, the transformation of the starch into 
sugar creates high osmotic pressures within the plant. Certain sugar- 
exuding glands, called extra-floral nectaries, are interpreted as safety 
valves for the relief of excessive internal pressures which might burst the 
cells of the plant or otherwise derange its physiological activities. 
