CHAPTER VII 



The Chemicdl Nature 

 of Growth Regulators 



Ever since the discovery of auxins, numerous workers have attempted 

 to find other new substances which would modify growth either in the 

 same manner as auxins or in some other manner. Perhaps one of the 

 first to sort through large numbers of compounds and mixtures of com- 

 pounds was Seubert (1925) who found that application of such things 

 as urine, saliva, and other materials to Avena coleoptiles elicit growth 

 responses. Since that time a host of other materials has been found to 

 possess biological activity and much confusion has arisen in interpret- 

 ing the different types of responses that have been obtained. While it 

 is not possible to establish an absolute classification of types of growth 

 responses that different substances will bring about in plants, certain 

 general categories can be recognized and, furthermore, one can easily 

 test for the characteristics on which the categories are based. 



By the definition given in chapter I, auxins are compounds which 

 can stimulate growth in the manner of the growth hormone. Auxins 

 can be distinguished from other substances which influence growth by 

 any growth test which utilizes auxin-free material. A common way of 

 testing for the property of stimulating growth is to utilize the slit 

 pea test or the Avena straight-growth test after washing the sections 

 for 2 to 4 hours as described in chapter II. The Avena test is an excel- 

 lent test for substances which are translocated readily. Substances 

 which show this property of growth stimulation are auxins in the true 

 sense of the word; that is, they themselves can substitute for the 

 growth hormone in causing growth. 



A variety of compounds other than auxins can stimulate growth, 

 though only in the presence of auxins. Compounds which seirve to in- 

 crease the effectiveness of auxin have been called synergists or in some 

 cases hemi-auxins. Other substances can bring about a swelling of cells 

 which may result in twisting and deformation of leaves or stems in- 

 stead of orderly elongation. These compounds are the epinastic agents. 

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