LIGHT AND VEGETATION 



135 



What is the process of this particular action of blue light? 

 Light can only cause the activation of certain chemical mole- 

 cules which absorb a light quantum. These activated mole- 

 cules then become capable of entering into reactions which 





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Phototropismj 



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* \ Photosynthesis 



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Visibility 



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Fig. 1, 1 5. Curves of relative eflBcacy of radiations for photo- 



tropism as a function of wave-length, according to Johnston, 



1934. Dotted curve: curve of visibility to the human eye. 



Broken curve: curve of efficacy for photosynthesis 



otherwise would be impossible for them, so that light may 

 have the effect, either of starting a chain of new chemical 

 transformations, or of transforming the sequence of reactions 

 at one of its stages and modifying the result. 



