CHAPTER XVII 



THE LIGHT RELATION 



ALL green plants exhibit direct relations to intensities 

 of light. The influence of light in the synthesis of the first 

 organic products, or photosynthates, has been considered ; 

 and it is now necessary merely to indicate some of the more 

 general ecological relations. 



261. The adjustment of plant members. No phenom- 

 enon of plant life is more familiar than the turning of leafy 

 shoots toward light or the orientation of leaves in a man- 

 ner to occupy a favorable exposure. Plants placed at the 

 window of a dark room promptly show the effects of the 

 light stimulus. The same relations may be observed in 

 the field. The capacity to show through growth curva- 

 tures an irritable response to light from one side is called 

 phototropism. We have to distinguish as main classes 

 of responding structures those axes which are parallelo- 

 tropic, curving in such manner that the tips point toward 

 or away from the source of light, and those which are 

 plagiotropic, or at some angle. Leaves are transversely 

 phototropic, and the response secures a favorable illumi- 

 nation of the chlorophyll bodies. As a result broad- 

 leaved plants develop commonly to form a more or less 

 perfect mosaic, no better examples of which can be found 

 than those of the grape-vine or Boston ivy. The adjust- 



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