14 GROWTH HORMONES IN PLANTS 



coleoptile). We shall put aside provisionally the question of whether 

 this differentiation is of a ' physical ' or ' chemical ' nature. The stimulus 

 is transmitted from the back side of the tip down the length of the back 

 side of the coleoptile. Since, as I have shown, there is no modification 

 in the rate of growth on the front side of the coleoptile, the positive 

 phototropic curvature must result from the accelerated growth rate 

 which is induced by the light stimulus." 



In his discussion of the nature of the transmission, Boysen 

 Jensen (1911) wrote further: 



"It seems to me that my studies of the transmission of the stimulus in 

 the Avena coleoptile render it probable that in this case the trans- 

 mission of the stimulus is of a material nature and produced by con- 

 traction changes in the tip of the coleoptile. In every case it would 

 seem necessary to resign oneself to a hypothesis according to which the 

 transmission of the stimulus in Avena would be due to physical causes 

 (changes of pressure, etc.), which is perhaps the case for Mimosa; in 

 fact we have seen that the stimulus can be transmitted across an 

 incision made in the coleoptile. For other reasons it is thought that 

 the transmission of the stimulus is of a chemical nature. As may be 

 recalled, the condition for transmission across an incision was that the 

 edges of the wound were kept humid and held one against the other in a 

 way to favor as much as possible transmission of a substance or of ions 

 across the incision. Another reason: It has never been proved that the 

 transmission of the stimulus could take place under water. The water 

 under such a condition should prevent this transmission, which can be 

 explained only in a hypothesis where the transmission of the stimulus 

 would have been due to the migration of a substance or of ions, which 

 would diffuse into the water and no longer act." 



It may be said, therefore, that in 1911, Boysen Jensen's 

 conception of phototropic curvature in the Avena coleoptile 

 was the following: Under the influence of unilateral light, a 

 polarity is formed in the coleoptile tip which is associated with 

 an unequal distribution of a substance upon the front and back 

 side of the coleoptile. The substance in question migrates down 

 the back side (he used the expression ''migration," since it 

 seemed clear, even at that time, that it could not be a process of 

 diffusion) and causes an acceleration of growth upon the back 

 side in the basal region, which produces a phototropic curvature. 

 This conclusion seemed the only possible one. The existence 

 of a growth substance in the Avena coleoptile during photo- 



