CHAPTER VIII 



THE SIGNIFICANCE OF GROWTH SUBSTANCES FOR 



PHOTOTROPISM 



When illuminated from one side, plant shoots usually turn 

 toward the light. This response is brought about by differential 

 growth, in such a way that curvature results. Though the 

 direction of movement is usually positive, there are some 

 instances in which organs turn away from the light; in such 

 cases, they are said to be negatively phototropic. Inquiries into 

 the nature of phototropism led to the discovery of substances 

 that regulate growth in plants; continued studies on this subject 

 have helped elucidate the mechanism of tropic responses. 



GENERAL DISCUSSION OF PHOTOTROPISM WITH SPECIAL 

 REFERENCE TO THE AVENA COLEOPTILE 



The benchng responses to light have been studied in many 

 kinds of plants, but the favorite object for investigations dealing 

 with the mechanism of phototropic curvature is the coleoptile of 

 Avena sativa. Certain distinct advantages in its use arise from 

 the fact that seeds from pure lines of the species can be obtained 

 practically every^^here, the plants can be grown easily in any 

 laboratory, and reproducible results can be obtained under 

 controlled environmental conditions. 



Stimulation and Response. The Light Gradient.— When an 

 Avena coleoptile is illuminated unilaterally, light is absorbed by 

 the tissues, and a descending gradient of light intensity across the 

 organ results. Under these circumstances, that part of the 

 coleoptile nearest the source of light naturally receives a greater 

 amount of light than the portion on the shaded side of the organ. 

 This gradient of the light stimulus, which is of considerable 

 importance for the theory of phototropism, has been measured 

 with great care by several investigators. 



Lundegardh (1922) tried to determine the magnitude of the 

 light decrease in unilaterally illuminated oat seedlings by employ- 

 ing photometric methods. It was found that the light in the 



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