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CHEMICAL AGENTS AND GROWTH 



AVERAGE INCREASE IN : 



STEM LENGTH NUMBER OF 

 (CM.) NODES 

 23 37 50 23 37 50 



23 37 50 '0 23 37 50 



Days after beginning of treatments 



Fig. 2. Effect of various defoliation treatments on the growth of 

 Weigela florida plants which have been shifted from long to short days at 

 time (adapted from Waxman, 1957). 



drawn: (1) short days stop growth in Weigela via a mechanism 

 present in the leaves; when all the leaves are removed, growth con- 

 tinues until the food reserves present in the stem and roots are ex- 

 hausted; (2) the young leaves, i.e., the ones which have reached from 

 one-half to three-fourths of their final size, are the ones which are most 

 effective in the photoperiodic control of growth and dormancy; the 

 older leaves have a much less pronounced effect. 



The experiment done with group E was an attempt to find out if the 

 regulatory principle made in the leaves under short days could be 

 transported from one branch to another. On each plant of group E, one 



