FORMATION AND OCCURRENCE OF AUXINS 61 



The formation of auxin in woody plants was first demon- 

 strated by van der Weij (1933a), who obtained auxin by 

 diffusion from the 3'oung leaves of Eleagnus. However, he 

 could obtain no auxin in this way from terminal buds of 

 young sprouting twigs. Czaja (1934), on the other hand, 

 obtained auxin readily by diffusion from sprouting buds of a 

 number of trees and shrubs, including Populus, Salix, Quercus, 

 and especially .4escM/7^s. W. Zimmermann (1936), also using 

 the diffusion method, has studied auxin production by buds 

 in some detail, and finds their behavipr to be closely similar 

 to that of buds of herbaceous plants studied by Thimann and 

 Skoog (see below). Production is greatest in the terminal 

 bud, and the more basal the lateral buds the smaller is their 

 production. In general he found a strict correlation between 

 the rate of growth of a shoot and the amount of auxin pro- 

 duced in its terminal bud, and thus concludes that in trees 

 development is largely dependent on auxin production. 



As to roots, the first evidence for auxin production in 

 the root tip came from the observation that root tips, like 

 coleoptile tips, could restore the geotropic sensitivity of 

 root stumps to which they were applied. However, it was 

 not until the experiments of Hawker (1932) and Boysen 

 Jensen (1933a) that direct e\adence of the presence of auxin 

 in root tips was obtained. They placed root tips on gelatin 

 (Hawker) or agar containing 10 per cent dextrose (Boysen 

 Jensen) and the resulting blocks were then able to cause 

 auxin curvatures on Avena coleoptiles. The same result was 

 obtained by Thimann (1934) by the extraction of root tips 

 with, chloroform (see Figure 30). The question as to whether 

 this auxin is actually produced in the root tip or not is 

 discussed in IX C. 



A more complete survey of the auxin production in the 

 entire plant in light was made by Thimann and Skoog (1933, 

 1934) on Vicia Faba (see Figure 28). The main center of 

 auxin production was found to be the terminal bud with its 

 embryonic leaves, but all the young leaves also produce 

 auxin to a lesser extent. The older leaves produce little or 



