AUXIN TRANSPORT AND POLARITY 



93 



(an apparent exception was described by Von Guttenberg 

 [1913], but this was afterwards contradicted). Hence the 

 phototropic stimulus is conducted only from the apex to- 

 wards the base. 



C. Polarity of Auxin Transport 



The experiments of Boysen Jensen (1910, 1911, 1913) 

 showed that the conduction of the phototropic stimulus 

 may take place across a cut surface and therefore involves a 



normal 



inverse 



I 



\J 



H S 



initial 



final 



initial final 



n 



i 



fs- 



light 



i 



i 



m 



I 



light 



A 



Fig. 37. Polar transport of auxin in the Avena coleoptile. I. Agar block, 

 A, containing auxin, is placed on coleoptile section, B, resting on plain agar 

 block, C. Degree of shading indicates auxin content of agar. Left side, normal 

 transport; right side, section inverted — no transport. (After Went, 193.56.) 

 II. Transmission of phototropic stimulus through introduced section of 

 coleoptile. Left side, stimulus passes normally placed section; right side, 

 does not pass inverted section. (After Beyer, 1928a.) 



diffusion process. The polarity of its movement led Paal 

 (1919) to suggest that the transport of the growth substance 

 in the coleoptile might itself be polar. The truth of Paal's 

 suggestion was proven by the experiments of Went (1928). 

 Since small amounts of auxin can be quantitatively analyzed 

 by the curvature technique, its movement through plant 

 tissues may be accurately followed. Went's technique was 

 as follows: on a number of sections of coleoptile (B in 

 Figure 37, I) was placed a block of agar (A) upon which a 



