PHOTOTROPISM JOHNSTON" 323 



The reason for such a claim is that there is no proportionality of 

 relationship between the very minute amounts of this substance used 

 and the enormous transformations that take place. It is also evident 

 that phototropism is directly related to the duration, intensity, and 

 wave length of light. With increasing understanding of these vari- 

 ous phases of the problem, we are getting a clearer picture of the 

 mechanism of plant growth and its bearing upon the welfare of 

 man and some of his economic problems. 



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I*RIESTLEY, J. H. 



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