94 PHYTOHORMONES 



number of coleoptile tips had previously stood, and which 

 therefore contained auxin. The concentration of auxin in a 

 control block prepared in the same way was determined 

 directly by the Avejia test. Underneath the coleoptile sec- 

 tions was placed a block of plain agar (C) and after they 

 had been in contact with it for some time, this, and also 

 the top block, were again analyzed by the Avena test. Thus 

 the original and final concentrations in the donating block, 

 A, and also the final concentration in the receiving block, C, 

 were known, and hence the amount which had been trans- 

 ported was found. Some auxin also was consumed, or in- 

 activated, during the experiment, but this had little effect 

 on the result. The principal conclusion was that transport 

 took place only from the apical to the basal end of the sec- 

 tion, and not inversely. It should be added that these 

 experiments were carried out with physiological concen- 

 trations of auxin. 



A confirmation of the fact that movement of the photo- 

 tropic stimulus within the coleoptile is strictly polar was 

 given by Beyer (1928a), whose experiments were an ex- 

 tension of those of Boysen Jensen. Between the tip and the 

 base of an Avena seedling he interposed a cylinder of coleop- 

 tile which in one group of experiments was in the normal 

 position, in the other group inverted (see Figure 37, II). If 

 the tip was now illuminated, the stimulus was transmitted 

 to the base, causing curvature there, but only through the 

 normally inserted section. 



Since it was now clear that the polar transmission of 

 stimulus is due to the polar transport of auxin, the nature 

 of this transport was subjected to an intensive investiga- 

 tion by van der Weij (1932, 1934). His technique was 

 essentially that described above, and qualitatively he con- 

 firmed the strict basipetal (apex-to-base) polarity of the 

 transport (Figure 38 C). If there is any acropetal transport 

 it is excessively slight (c/. Snow, p. 97). As regards the 

 quantitative aspects, he found it necessary to distinguish 

 between the velocity, in mm. per hour, and the ''intensity" 



