GENERAL CONCLUSIONS 249 



all findings with pure substances. However, the recent 

 increasing use of these substances in unphysiologically high 

 concentrations constitutes a danger, because the results do 

 not necessarily have any bearing on the functions of auxin 

 in the normal plant. Lastly, until quite recently it has been 

 customary to investigate as far as possible every point 

 bearing on a theory before the theory was enunciated, so 

 that most of the views reached were well-founded and could 

 be used as basis for further work. In the last year or two 

 this procedure has not been so rigidly adhered to; examples 

 of generahzations without sufficient experimental foundation 

 are furnished by the comparison between the action of 

 ethylene and auxin discussed in XI C, by the theory of the 

 role of acids in producing growth directly (VIII F), by the 

 interpretation of experiments on the movement of auxin 

 in the transpiration stream (VI D), and by the two-stream 

 theory of inhibition by auxin (VIZ), IX D). There is 

 danger, in any rapidly developing field, of an accumulation 

 of unclassified facts and unproven theories which makes 

 further development much less certain. In the field of 

 phytohormones this is particularly unjustifiable, because 

 the experimental procedure is relatively simple and the 

 equipment necessary is not too elaborate. If the criteria of 

 high-class experimental work continue to be observed, then 

 we may look forward in the next few years to the rapid 

 solution of a great many of the interesting problems of 

 growth and development. 



