CHAPTER XVIII 



Potentials of Auxins and 

 Auxin Research 



With the continued development of the auxin field it has become 

 more and more evident that the concept of the growth hormone in 

 plants must be expanded to a concept of the hormonal control of 

 growth, development and metabolism. The streams of flow of the 

 growth hormone from each of the plant extremities (figure 1) control 

 not only the growth characteristics of the plant, but they also control 

 the differentiation of tissues and organs of various sorts, they play a 

 multitude of intimate roles in the reproductive functions of flower 

 initiation, pollination, embryo development, et cetera, and they deter- 

 mine to a large extent the morphological constitution of the plant as 

 an organism. The growth hormone, then, appears to be a major in- 

 fluence in determining the characteristics and development of the 

 plant as an organism. In applying auxins to plants, the myriad of 

 growth hormone actions is being altered artificially, and because of 

 the immense number of natural functions of the growth hormone in 

 plants, auxin applications can have an immense number of different 

 uses. 



THE QUANTITATIVE ASPECT OF AUXIN EFFECTS 



In looking back over the many and heterogeneous effects of auxin 

 on growth and development of plants, it is strikingly evident that 

 different quantities of auxin can produce entirely different or even 

 opposite effects. Thus growth can be promoted or inhibited by auxin. 

 Tropisms can be positive or negative depending upon sensitivity to 

 auxin. Abscission and flowering can be accelerated or retarded depend- 

 ing on the location of the auxin and other factors. At the cellular 

 level either differentiation or dedifferentiation can be induced by 

 auxin. 



Each phenomenon in growth and development which is affected 

 by auxins has its own concentration optimum, so that a given auxin 

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