102 Fundamentals of Auxin Action 



demonstrated that, at least in the coleus, the appearance of apical 

 flowers is associated with a strong alteration of the polarity pattern 

 in a stem, it is tempting to speculate that the associated increase in 

 branching may be due to a less active polar transport of auxins from 

 the apex (Leopold and Guernsey, 1953). 



A contrast may be drawn between the role of auxin in dormancy 

 and its role in apical dominance. In the former case, there appears to 

 be an insufficient supply of effective auxin for growth because of the 

 presence of antagonists which can interfere with the normal function 

 of whatever auxin is present (Hemberg, 1947). In the latter case lateral 

 buds are prevented from developing by reason of the high, inhibitory 

 levels of auxins moving past the buds or, at least, present in the stems 

 adjacent to them. 



The possibility that auxins connnonly exist in roots at levels 

 which are inhibitory to root growth has been evident for many years. 

 The early experiments by Cholodny (1929) support this concept ad- 

 mirably. He found that decapitation of roots of many different kinds 

 of plants could result in an increase in subsequent root elongation. 

 Not only that, but the replacement of the root tip would again reduce 

 the rate of growth. This evidence strongly suggests that auxins pro- 

 duced in the root tip are at a super-optimal concentration for root 

 growth. Some exceptions to this inhibitory situation have been men- 

 tioned in the previous section. A common exception is found in the 

 pea, reported as early as 1936 by Thimann. He found that pea roots 

 could be stimulated by the addition of auxin and inhibited in growth 

 rate by decapitation. The capacity of pea roots to respond positively 

 to further additions of auxins has been utilized in a highly sensitive 

 bioassay for auxin (see chapter II). 



ORGAN DIFFERENTIATION 



Soon after the discovery of the growth hormone it was found that 

 the hormone and other auxins affected not only growth by elongation, 

 but also affected the morphological type of growth obtained. Thus an 

 application of auxin to a yoimg stem might result in a callus type of 

 growth. Also, stem cuttings treated with auxin could be induced to 

 form roots. More recently it has been discovered that auxin levels play 

 a determining role in the differentiation of many types of meristems 

 including roots, buds and sometimes even flowers. In time, therefore, 

 it has become apparent that auxins participate in a general control 

 of organ differentiation. By some means as yet unknown, auxins are a 

 factor in determining whether a cluster of cells will be differentiated 

 into callus, roots, vegetative buds (stems and leaves), or flower buds. 



