66 KENNETH V. THIMANN 



of geotropic accumulation of enough extra auxin on one side to start 

 the flowering process. 



In summary we may conclude that growth depends not only on the 

 free auxin present but also on the ability of the tissue to convert bound 

 auxin into the free form. The bound auxin is itself of many types : ( i ) 

 that of the seed and perhaps the ovary which is made available for 

 growth, (2) that of the leaf which is not, and (3) that of the main axis, 

 which may occupy an intermediate position, while (4) that in the 

 coleoptile appears much more loosely bound than the types discussed 

 above. 



I come now to the mechanism of the action of auxin. One of the 

 greatest values of our knowledge of the auxins is that they provide a 

 tool for the analysis of the growth process itself. I will try to outline 

 briefly the progress made in this direction, but first I want to make the 

 point that all this evidence that auxin can combine with proteins, and in 

 many different ways, has another significance. From studies of the 

 structure of the many substances active as auxins, Koepfli, Went and I 

 have earlier deduced (1938) that there is a certain spatial relationship 

 between the various parts of the molecule which is essential for activity. 

 This suggests that the auxin has to combine with another molecule 

 whose spatial structure is fixed, such as a protein, for example. This 

 leads naturally to the idea that the auxin is a coenzyme for some enzyme 

 system which controls growth. 



It has been known for a long time that growth of the coleoptile will 

 not take place anaerobically, and Bonner showed that growth is inhibited 

 by cyanide to the same degree as respiration is. This suggests that 

 growth is in some way linked to respiration, but it does not provide a 

 mechanism for studying it. However, Commoner and I (194O sub- 

 sequently found that various known inhibitors of dehydrogenase sys- 

 tems also inhibit growth. This led to a study of the whole relationship 

 between enzyme inhibitors and growth. 



The technique was the straight growth of isolated coleoptile sections, 

 which grow little in water or sugar alone, more in auxin, and consider- 

 ably more in the combination of auxin and sugar. Growth was in 

 shallow layers of solution by placing the sections on the teeth of combs. 



Of all the inhibitors studied, iodoacetate was one of the most effec- 

 tive, giving complete inhibition at about 3.10"^ molal, an exceedingly 

 low concentration for most enzyme inhibitions. Under these conditions 

 the respiration of the sections was very little reduced. We have here not 

 an effect on the general vitality or integrity of the cells, but specifically 



