I 



GROWTH HORMONES — THIMANN 397 



grow and soon becomes the leader. In other words, this bud was 

 capable of growth all the time, but did not do so because the terminal 

 bud was present, i. e., it was inhibited by the terminal bud. Now 

 since the terminal bud produces relatively large amounts of auxin, 

 Skoog and I made the experiment (1934) of removing this bud and 

 putting in its place a supply of auxin. The buds below were then 

 inhibited to the same extent as they would have been by the terminal 

 bud. The auxin which this bud produces, then, has two functions: 

 it causes the stem below it to grow, and it causes the buds below it 

 to be prevented from growing. This is the first example of what 

 was later found to be very general, namely, that auxin elicits differ- 

 ent responses from different plant parts. Although normally it 

 causes the stems to grow while the buds are inhibited, it must not 

 be thought that this is simply a balance or a compensation of growth, 

 i. e., that if the stem grows the buds do not and vice versa. For we 

 found that it is possible by using the right conditions to prevent the 

 growth of the buds without causing the stem to increase appreciably 

 in length. In other words, the inhibition is quite independent of 

 other growth processes. 



At the same time that this work was done we were engaged in the 

 study of another problem. It is known that isolated parts of stems, 

 i.e., cuttings, form roots under certain conditions. Generally cuttings 

 root better if young buds or leaves are on them. On this account 

 it was thought possible by van der Lek and by Went (1929) that 

 the formation of roots is controlled by a hormone. Went and I 

 (1934) soon found that certain preparations when applied to cuttings 

 which otherwise were not in the condition to root (having been kept 

 in the dark) caused rooting, and that the number of roots formed 

 could be made roughly proportional to the concentration of the ma- 

 terial used. Work was therefore begun on the purification and iso- 

 lation of the active root-forming hormone. It was not long before 

 it became clear that the richest sources of this were the same materials 

 which had proved the rich sources of auxin, namely urine and the 

 cultures of fungi already mentioned. On successive stages of puri- 

 fication of the root-forming hormone, its activit}'^ always went along 

 with the auxin activity. Finally we became convinced that the root- 

 forming hormone is identical with the growth hormone, auxin. Tliis 

 was proved when we synthesized an auxin (indole-acetic acid, which 

 Kogl and Haagen Smit had just isolated from urine and shown to 

 be an auxin) and found it to be highly active in producing roots. 

 Furthermore, the production of roots, like the promotion of growth, 

 is general and not limited to special groups of plants, so that the 

 use of auxins by nurserymen in promoting the rooting of cuttings has 

 since that time become widespread. Several synthetic auxins have 

 been marketed for this purpose. 



