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PREFACE TO THE GERMAN EDITION 



Twenty-five years have passed since the discovery of growth 

 substance in the Avena coleoptile. The development of growth- 

 substance research in this period of time is characterized by 

 a continually growing mass of publications. The growth- 

 substance literature, in a narrow sense, includes at present about 

 200 individual papers. Investigations in this field are appear- 

 ing continually, so that important new discoveries must be taken 

 into consideration each year. One can foresee great forward 

 strides during the next few years in this line of study. 



What has been accomplished so far by this extensive activity? 

 The most important contributions are as follows: The signifi- 

 cance of growth substance for photo- and geotropic curvatures 

 was demonstrated first in the Avena coleoptile. Then its effect 

 on normal growth was investigated, and it was shown that 

 growth substance plays an important part in the positive 

 geotropic curvature of the main root. Next the general occur- 

 rence of growth substances was demonstrated in higher and 

 lower plants and in animals. Finally it became possible to 

 prepare growth substance in a pure state. The main result has 

 been a glimpse into the mechanism of the orientation of higher 

 plants in space. 



Do the results that have been obtained justify the effort — has 

 growth-substance research had its reward? Perhaps such a 

 question should not be asked. I feel that it would be unfair 

 to suppress the fact that, although great advances have been 

 made, we are still far from an explanation of photo- and geotropic 

 processes. It is satisfying to know that something has been 

 attained that may be regarded as progress in our knowledge 

 of life processes. One must become reconciled to the fact that 

 in advanced physiological research, problems become so involved 

 that every stride forward requires a great output of labor. 



The author has attempted to present the chief lines of investi- 

 gation and the main results of research done in the last twenty- 



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