206 PHYTOHORMONES 



Layering probably depends for its success on the retarding 

 influence on auxin transport exerted by high humidities 

 {cj. XIII C), together with geotropic accumulation of auxin 

 on the lower side of the stem. Rooting takes place usually 

 at nodes, probably because there the transport of auxin is 

 interfered with. {Clematis is an exception, rooting better at 

 internodes.) The practice of ringing branches, either by 

 cutting the cortex or by tying a tight wire round during 

 growth, obviously operates in the same way. The optimum 

 time of year to take cuttings varies from plant to plant 

 (Graham, 1934) and depends upon a number of factors 

 such as auxin production, storage, and destruction, as well 

 as water supply and ease of wilting. 



Among the causes of failure of cuttings to root when 

 treated with auxin, one of the most important is doubtless 

 furnished by those plants in which not auxin, but one of the 

 other factors, is limiting. Another cause is the loss of the 

 applied auxin or other factors by exudation from the cut 

 surface; cuttings which root with difficulty are frequently 

 those from which much exudation takes place. Lastly, the 

 inactivation of auxin at the cut surface, by enzymes set 

 free in wounding, doubtless also plays a part. 



In conclusion, it may be pointed out that the role of 

 auxins in root formation is a good example of a piece of 

 research in pure physiology which has an immediate practi- 

 cal application. 



