CHAPTER VI 



AUXIN TRANSPORT AND POLARITY 

 A. Auxin Transport in General 



The Avena test method for measuring the concentration 

 of auxin in agar blocks makes use of the ready transport of 

 auxin within the coleoptile. Practically every auxin ex- 

 periment reveals how easily the plant allows the rapid 

 movement of auxin from one place to another. On this 

 transportability of auxin depends its function as a hormone 

 or correlation carrier. 



The rate of auxin transport in the plant is far greater 

 than that of diffusion, and, as will be seen later, van der 

 Weij (1932) has shown that many properties of the trans- 

 port process rule out the possibility of its being a diffusion 

 phenomenon. In the intact coleoptile the rate of auxin 

 transport is of the order of 15 mm. per hour, in the cut 

 coleoptile slightly less, — 10-12 mm. per hour. The rate 

 also differs for different auxins, auxin a being transported 

 about 10 per cent faster than indole-acetic acid {u; cf. VIII G). 



As to the path of transport of auxin, few data are avail- 

 able, and, as is always the case in the absence of good data, 

 conflicting views are held. It is generally assumed that in 

 the Avena coleoptile transport takes place through all parts 

 of the parenchyma, and not primarily through the vascular 

 bundles. If the latter were the case, no well oriented curva- 

 tures towards light or gravity could be expected, because 

 there are only 2 bundles present. Further, if an agar block 

 containing auxin is placed on the cut surface of a coleoptile 

 stump, a curvature is obtained whether the agar covers a 

 vascular bundle or not. Van der Weij (1932) states that the 

 curvature is stronger when it does not, but Laibach and 

 Kornmann (1933a) find the reverse (cf. also Snow, VI C). 

 In our own experiments (u), no important difference be- 



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