Oct. i Sl 1920 Influence of Cold in Stimulating Growth of Plants 1 59 



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 constructed in 191 2. It was kept 

 above freezing by heating with electric lights, which were turned on and 

 off 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 was crude and inade- 

 quate. To provide for more exact experiments a glass-covered com- 

 partment chilled by a refrigerating machine was constructed in one of 

 the Department of Agriculture greenhouses. The refrigerating appa- 

 ratus is a sulphur-dioxid 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 compartment, 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 con- 

 structed in 1 91 6. In this enterprise I had the valued advice and assist- 

 ance of Dr. Lyman J. Briggs. The usefulness of this refrigerated frame 

 in experimental work with plants was so great that another similar 

 equipment was installed in 1918. 



With the aid of this apparatus many of the experiments described in 

 this address have been carried on or verified, as well as other experiments 

 of a related character. For example, at ordinary summer temperatures 

 many kinds of seed will not germinate but remain dormant until death 

 overtakes them. Under the influence of chilling, however, these seeds 

 are stimulated to prompt germination. (See PI. 29.) 



The experiments thus far made indicate the importance of a much 

 wider use of the principle of chilling in many lines of experimentation 

 bearing on the improvement of horticultural and agricultural practices. 

 I commend the subject of chilling to experimenters in these lines, and 

 I wish to call especial attention to the desirability of determining" proper 

 temperatures for the storage of seeds, bulbs, cuttings, and grafting 

 wood, proper temperatures for the treatment of plants which are to be 

 forced from dormancy to growth at unusual seasons, and proper tempera- 

 ture for the storage of nursery stock so that the nurseryman may have 

 plants in proper condition for shipment on any date he desires. (See 

 PI. 30; 31, B; 32.) 



The whole question of the effect of chilling on herbaceous perennials 

 is an open field. 



An understanding of the process of chilling explains the reason of 

 some of the practices of gardeners, which they as well as botanists have 

 erroneously ascribed to the need of "resting." What a gardener calls 

 "resting" is often in reality a period of chilling, characterized not by 

 physiological rest but by pronounced internal activity. Rest alone would 

 not, in the case of our cold-climate trees and shrubs, accomplish the 

 purpose the gardener has in mind. It is chilling, not rest merely, that 



