Oct. is, 1920 Influence of Cold in Stimulating Growth of Plants 1 53 



the rest of the winter the plant remained in the same position, the pot 

 and the stem, shown at the left in the illustration, continuing in the 

 warm temperature of the greenhouse, while the stem at the right, 

 projecting through the glass, was exposed to the rigors of winter, 

 with its alternate freezing and thawing. The illustration, from a photo- 

 graph made April 18, shows that when spring came the outdoor branch 

 started into normal growth while the indoor branch continued dormant. 



A second illustration (PI. 23) shows a modification of the first experi- 

 ment. In this case the plant was set on a shelf outside the greenhouse, 

 and a single branch was passed through the glass wall into the warm 

 interior. When spring came it was this interior branch that remained 

 dormant, all the outside branches putting out leaves promptly and 

 normally. 



From a comparison of the two experiments it is evident that the 

 difference in behavior of the indoor and outdoor branches could not 

 have been caused by any special action of the root system, for in one 

 experiment the roots were inside, in the other outside. It is clear that 

 the causes that stimulated growth in the exposed stems operated in the 

 stem itself, not in the roots. This principle is still further exemplified 

 and confirmed by the behavior of cuttings taken from blueberry plants 

 in the first stages of their dormancy. Such cuttings if kept warm con- 

 tinue their dormancy into late spring or summer, but if chilled for two or 

 three months they start into growth at the normal time in early spring. 



It should be stated here that the difference in the amount of light 

 inside and outside the greenhouse had nothing to do with the stimulation 

 to growth, for chilled plants are ready to start into growth promptly 

 whether the chilling is done in the full light of an outdoor situation, or 

 in the partial light of a greenhouse, or in the complete darkness of an 

 ordinary refrigerator. 



4. The stimulating effect produced on dormant plants by cold 

 is intimately associated with the transformation of stored 

 starch into sugar. 



In most of our wild species of trees and shrubs the reserve carbo- 

 hydrate material is stored away during summer and autumn in the form 

 of starch. At the beginning of dormancy the twigs and sap wood are 

 gorged with this material, the starch grains being stored ordinarily in the 

 cells of the medullary rays and sometimes in the pith. As the process of 

 chilling goes on, this starch little by little is transformed into sugar. 

 The presence of large quantities of starch in the fall and early winter 

 may be observed by applying to freshly cut surfaces of the twigs the well- 

 known starch test of a 2 per cent solution of iodin in a 1 per cent solution 

 of iodid of potassium. With a strong hand lens the starch is readily 

 observed, if present, by the deep blue color it assumes under this treat- 

 ment. The intensity of the coloration gives roughly an idea of the 

 187932°— 20 6 



