CHAPTER XIV 

 GENERAL CONCLUSIONS 



In the preceding pages we have seen how from analyzing 

 the complexities of correlation and of tropisms there has 

 emerged a clear concept of hormones and of the role they 

 play in the plant. It could hardly be expected, however, 

 that, after only about 10 years of research this concept would 

 have led us to a finished explanation of all the processes 

 in which auxin is involved. Still less could we be expected 

 to have progressed in our knowledge beyond this one group 

 of hormones, the auxins. Our knowledge of the auxins has, 

 as a matter of fact, laid open to experimental attack many 

 problems concerned with other factors in the plant. How- 

 ever, it is fair to say that we already see the auxins and their 

 properties as a continuous thread connecting most of the de- 

 velopmental and growth processes in the plant. Thus the 

 auxins bring tropisms into that close relation with growth 

 upon which Blaauw insisted in 1918; they bring growth, in 

 the general sense of development and organ formation, into 

 the same terms as growth in the special sense of cell elonga- 

 tion; and lastly, they bring the concept of correlation, which 

 by its very name has previously defied causal analysis, into 

 the realm of direct experimental attack. Perhaps the best 

 example of this last phase is the meaning which is now given 

 to the formerly elusive conception of polarity; polarity can 

 now be expressed quantitatively as a function of the transport 

 of a known substance in the tissues. Finally, few fields can 

 have benefited so much from close interaction between the 

 biological and chemical approaches, and the remarkable dis- 

 coveries on the chemical side have made possible equally 

 remarkable progress in the physiology. It has even been 

 possible to inquire somewhat into the inner mechanism of 

 the relation between auxins and their substrate. 



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