2 PHYTOHORMONES 



idea that correlations in plants are due to the influence of 

 special substances (Chapter II), ^ We shall try to show how 

 experiments along four different and apparently unrelated 

 lines, — correlation proper, organ formation, tropisms, and 

 normal growth, — have gradually come together and been 

 unified into a complete picture of hormone activity as we 

 now know it. Next we shall consider the methods for the 

 assay of these substances, treating them in sufficient detail 

 for experimental use (Chapter III). Since most of these 

 methods are founded upon cell elongation, and all other 

 work has had its foundation thereon, it is natural to con- 

 sider cell elongation in some detail first. 



The best demonstration of the effectiveness of these 

 assay methods has been the working out of the chemical 

 nature of the active substances (Chapter VII). The success 

 of the chemical attack has made it possible for all the exper- 

 iments described subsequently to be checked by use of the 

 pure compounds ; this has had the effect of making the con- 

 clusions clear-cut and has avoided the difficulty of working 

 with unknown extracts and mixtures. 



Parallel and simultaneous with work on the above lines, 

 the role of the active substances in various aspects of plant 

 growth has been elucidated (Chapter V), beginning with 

 their formation (Chapter IV) and movement in plant tissue 

 (Chapter VI). The latter phenomenon is of special interest, 

 firstly because it offers an example of a naturally occurring 

 substance whose movement can be followed quantitatively 

 throughout the plant, and secondly because of the causal 

 relation between the polarity of this transport and the well- 

 known polarity of plant structures. One of the most inter- 

 esting aspects of the subject has been the attempt to analyze 

 the various reactions interv^ening between the auxin and its 

 final effect, — growth (Chapter VIII). In this, knowledge 

 of the chemical nature of the substances has played an essen- 

 tial part. In close connection with problems of cell elonga- 



1 References such as VIII G or III C refer to chapter (Roman numbers) and 

 section (letters). 



