66 Fundamentals of Auxin Action 



ble auxin of young bean leaves is very fiigfi indeed until they reach 

 the age of twenty days, when growth is essentially complete. The 

 production of fairly constant amounts of auxin over the following two 

 months' period demonstrates a possible exception to the correlation of 

 auxin production with growth rate. 



All leaves do not show the same picture of auxin production as 

 seen in bean leaves. While coleus, aster, bean and goldenrod seem to 

 follow such a pattern, auxins have not been found in expanding leaves 

 of the ginkgo (Gunckel and Thimann, 1949). It is not clear if the 

 failure to find the usual pattern of auxin production is due to the 

 presence of growth inhibitors in the leaf which interfere with auxin 

 assays. 



There is evidence that the apex of the woody plant loses its role 

 as the primary auxin source shortly after twig elongation begins. A 

 typical succession of events in the auxin production of a twig might 

 be as follows: a) in the dormant bud there is essentially no diffusible 

 auxin; b) at the time that swelling of the buds commences, a rela- 

 tively large amount of auxin appears in the bud; c) as elongation of 

 the internodes begins, auxin production by the tip reaches a peak. In 

 cases where twigs do not elongate, as in the short shoots of ginkgo or 

 spurs of fruit trees, this peak is not very pronounced; d) as elongation 

 proceeds, the apex becomes poor in auxin and each internode is appar- 

 ently supplied with auxin primarily by the region of the node above 

 it; e) as growth ceases, with maturity of the entire twig or an individual 

 internode, auxin content apparently remains high for a brief addi- 

 tional period. These stages of auxin production in woody shoots can 

 be seen in the data of Zimmerman (1936), Avery et al (1937), Gunckel 

 and Thimann (1949), and Hemberg (1949). 



One would expect that the growth rate should correlate with the 

 growth hormone content, and that the growth response to applied 

 auxins should likewise correlate with the amount of auxin applied. 

 The response to applied auxins is in fact proportional to the amount 

 of auxins applied (see chapter IV). The degree of responsiveness 

 varies greatly with the type of tissue, and the type of response also 

 varies with a variety of physiological factors, but the correlation be- 

 tween growth and auxin holds under a wide diversity of situations, 

 both natural and artificial. 



FREE AND BOUND AUXINS 



From the time that physiologists first began studying the results 

 of long term diffusion experiments and long term extraction experi- 

 ments it has been evident that auxins in the plant do not all exist 



