PHOTOTROPISM 465 



light of different intensities, curvature will occur only when one light 

 source is 1.20 times as intense as the other (Pringsheim, 192G). To 

 explain this, we have to admit that the sensitivity to light is reduced by 

 hght itself ; thus the tonus, or Stimmiing, of the coleoptile decreases as it 

 is subjected to a greater amount of light. This was analyzed in detail 

 by Arisz (1915). 



Lange (1927) and Sierp and Seybold (1926) have measured the sensi- 

 tivity of different regions of the coleoptile to hght in numerous experi- 

 ments. They exposed the extreme tip or narrow transverse bands of 

 the coleoptile to unilateral light and determined the amount necessary 

 to cause a just visible curvature. They found that the extreme 0.1 mm 

 of the coleoptile tip was the most sensitive to light. From that point 

 the sensitivity dropped about 5000 times to a rather constant value for 

 all regions more than 2 mm distant from the tip. Therefore, when a 

 coleoptile is illuminated with a sufficiently large amount of light, the 

 basal cells will also respond directly to light. We should therefore dis- 

 tinguish between the tip response and the base response in the Avena 

 coleoptile. These differ, first, in the amount of light necessary to pro- 

 duce a response and, secondly, in the location of perception of the light. 

 There is transmission of the stimulus in the tip response but no such 

 transmission in the base response, since the illuminated cells in the base 

 respond directly. The third difference is significant in this connection: 

 the base response occurs within half an hour after illumination, v/hereas 

 it takes almost an hour before a curvature starts to develop as a result 

 of the tip response. As a fourth difference might be mentioned the work 

 of Haig (1935) in which a slight disparity in spectral sensitivity was found 

 between the tip and the base response. 



Between 1914 and 1918 Blaauw (1914, 1915, 1918) formulated his 

 theory of phototropism, stating that a phototropic response results when 

 local differences in growth rate are produced through local differences in 

 light intensity. He based this theory on extensive series of measure- 

 ments in which he showed that the growth of any phototropically sensi- 

 tive organ is either retarded or accelerated by Hght. Therefore, when 

 an organ is subjected to unilateral illumination of unequal intensity on 

 either side, these local differences in light intensity produce local differ- 

 ences in growth rate, which in turn account for the phototropic curvature. 

 This theory was hotly contested, and during the 1920's scores of papers 

 appeared, either for or against Blaauw's theory. 



Blaauw's theory does not account for a transmission of stimulus from 

 tip to base. A new theory was formulated, almost simultaneously, by 

 Cholodny (1927) and Went (1928a). This theory is based on the fact 

 that the lower zones of the Ave7ia grow under the influence of the growth- 

 promoting substance auxin, which is formed in the coleoptile tip. The 

 auxin is normally distributed equally over the different sides of the 



