AUXIN TRANSPORT AND POLARITY 



103 



scheme would allow for transport with high capacity and 

 velocity, being many hundred times faster than diffusion. 

 However, at the moment there seems no acceptable way of 

 associating it with any polarity of movement. 



All these schemes, though they may explain the type of 

 movement observed, are without any bearing on the essential 

 problem, namely that of 

 the polarity. Directed 

 movement of a substance 

 in solution can only be 

 due to (1) a pressure 

 gradient, (2) a concen- 

 tration gradient, or (3) an 

 electric potential gradi- 

 ent. TjTDes (1) and (2) can 

 be discarded because no 

 pressure gradient exists, 

 and because transport 

 takes place independent 

 of, or even against, con- 

 centration gradients. Po- 

 tential gradients could 

 only cause the movement 

 of ions, and so far all 

 the auxins known are ion- 

 izable substances. Went (1932) has tried to base a theory of 

 polarity upon these considerations and upon some experiments 

 with dyes. It was found that acid dyes moved in plant 

 tissues more rapidly toward the base than toward the apex, 

 while basic dyes moved in the reverse direction. This shows 

 that direction of movement is determined not merely by 

 chemical constitution, but rather by the dissociating groups. 

 Correspondingly, of the numerous measurements of electric 

 potentials in plants which have been made in the past, the 

 majority show that the apex of the shoot is negative to the 

 base, a potential in the right direction for transport of an 

 anion, such as of auxin, towards the base. However, in such 



Fig. 39. Model to show rapid trans- 

 port of surface-active substances along £n 

 interface. Horizontal tube ab, 100 cm. 

 long from d to e, is half filled with very 

 dilute acid and half with ether, and chlor- 

 phenol red is added. When a little potas- 

 sium oleate solution is added at a the in- 

 dicator changes to purple, the color change 

 moving rapidly along to b, where it can 

 be titrated with acid, using the stirrer kij. 

 Thus there is mass movement of potas- 

 sium ions along the interface. (From van 

 den Honert, Proc. Kon. Akad. Wetensch. 

 Amsterdam 35: 1104-1111, 1932.) 



