PREFACE 



It is with enthusiasm that we bring a translation and revision of 

 Professor Boysen Jensen's "Die Wuchsstofftheorie " to botanists 

 in English-speaking countries. It is the first comprehensive 

 review of the literature dealing with the role of growth hormones 

 in normal growth and tropisms of plants. In translating and 

 revising the book to include the 1935 literature, it is our hope to 

 stimulate sound progress in this important new field of plant 



physiology. 



Plant growth hormones have been recognized by many investi- 

 gators as offering fruitful opportunities for study, and there is 

 ample evidence in the contributions of the past year or two that 

 research in this field is progressing at a rapid pace. One dis- 

 covers in the literature that growth hormones promote cell 

 enlargement in shoots of higher plants, initiate the development 

 of roots (but at the same time inhibit their growth in length), 

 inhibit the development of lateral buds, stimulate cell division 

 in the cambium, and bring about callus formation. Such diverse 

 effects are of great interest, but in many instances more evidence 

 must be obtained to prove the theoretical views which we now 

 possess. Professor Boysen Jensen's review of the subject makes 

 clear numerous weaknesses and gaps in our knowledge and thus 

 helps point the way for future research. 



The arrangement of certain chapters has been modified in 

 the translation so that students not already acquainted with the 

 literature may become familiar with techniques and general 

 methods of procedure. Certain parts of the book have been 

 condensed slightly, while others have been expanded to include 

 data from the approximately 200 new citations added to the 

 bibliography. The essential features of the controversial final 

 chapter of the German edition have been included so far as 

 possible in earlier chapters. In an attempt to make the American 

 edition as useful as possible to students, we have added an index, 

 a summary at the end of each chapter, and numerous illustrations. 

 The following new figures have been inserted: 7, 8, 13-15, 17-20, 



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