viii PREFACE 



23-26, 28-31, 33-44, 50, 55, 58-62; and Figs. 21 and 22 have 

 been substituted for Figs. 10 and 11 of the German edition. 

 The historical development of the subject has been presented 

 in a series of diagrams (Figs. 1 and 2). Plant growth substances, 

 that is, Wuchsstoffe, have been referred to by various workers 

 as growth hormones, growth regulators, growth enzymes, phyto- 

 hormones, and auxins. We have used all of these terms. A 

 really satisfactory terminology, based on the chemical nature 

 of the compounds in question, will have to wait until more is 

 known. 



A selected list of titles dealing with the influence on plants of 

 substances such as bios, folliculin, and other sex hormones, those 

 affecting the growth of fungi, etc., has been added for the con- 

 venience of students interested in these topics. They are not 

 discussed in the text. 



It is a pleasure to express our appreciation of Professor Boysen 

 Jensen's interest and cooperation in the effort to make this book 

 available to English-speaking botanists. 



George S. Avery, Jr. 

 Paul R. Burkholder. 



Connecticut College, 

 June, 1936. 



