FORMATION AND OCCURRENCE OF AUXINS 69 



Dijkman (1934) found that by placing sections from 

 young Lupinus hypocotyls on agar, about the same amounts 

 of auxin were obtained from the upper, middle, and lower 

 zones. In older plants, the auxin concentration decreased 

 towards the base. 



Avery (1935) found that in tobacco leaves, the veins 

 contain auxin although presumably they do not produce it; 

 the auxin appears to accumulate in the veins towards the 

 base of the leaf. 



It is clear from what results are so far available that auxin 

 is present almost throughout the plant, although in highest 

 amounts near to where it is produced or stored. 



To account for the presence of auxin far below the grow- 

 ing regions (in which it is known to be used up in the growing 

 process), Soding (1936a) assumes that although cells need 

 auxin for their growth, they also form it during growth, 

 and even give it off. The arrival of auxin at one end of a 

 cell would stimulate the other end to form some auxin. ^ 

 Thus auxin formation would be a typical response to stim- 

 ulus, and Soding concludes: auxin is not only the cause, but 

 also the result of growth. However, in none of the very 

 numerous experiments on transport has more auxin ever 

 been collected from the basal portions than had been admin- 

 istered apically. In a few cases the original auxin content of 

 the tissues has added to the amount found, but this can be 

 determined from blank transport experiments, in which no 

 auxin is applied. Further, we have seen in IV A that in 

 darkness the auxin content of a plant (other than a seedling) 

 continually^ decreases, so that there is, in toto, no new forma- 

 tion although there is growth. If it is further remembered 

 that auxin production is not by any means limited to grow- 

 ing parts, it will be clear that Soding's assumption is un- 

 justified. 



The facts can more usefully be looked at in the reverse 

 direction. Naturally if a region of the plant begins to produce 

 auxin it will also begin to use that auxin for growth, providing 



* Cf. Stanley's suggestion (1936) for the movement of the virus protein. 



