Growth Substances 



383 



*ft 





I 



I 



7.8 



5.4 



9.2 

 10.0 



7.3 

 6.8 



8.2 



2.7 

 78 



6.2 



4.0 

 3.5 



0.0 



0.0 



Fig. 18-7. Concentration gradient of growth hormone (in terms of Avena curvature) 

 at various levels in a twig of Aesculus on May 16, when the growth of the current 

 year ( above arrow ) was just completed. ( From Avery, Burkholder, and Creighton. ) 



GROWTH SUBSTANCES AND CORRELATION 



Of much significance for morphogenesis is the role of growth sub- 

 stances in the correlation of growth (Thimann, 1954/?). Here is involved 

 the whole problem of organized development. These substances seem to 

 be the agents by which many such correlations are achieved, but one 

 should hesitate to call them "growth regulators" since the actual regula- 

 tion must go further back, to the mechanism that controls their distribu- 

 tion and local concentration and binds the parts and processes into an 

 organism. 



Differential Movement. An important fact about auxin is that in living 



