Preface 



THE concept of hormonal regulation of growth dates back nearly a 

 century to Julius Sachs, who deduced from his experiments on 

 plants that special substances are responsible for the formation and 

 growth of different organs. However, definite proof of hormonal action 

 in plants was not obtained until 1926-28 when Went demonstrated a 

 growth substance, auxin, in the tip of the oat seedling. This discovery 

 marked the culmination of a long period of quantitative experimentation 

 on the nature of plant movements and the beginning of a new approach 

 to the study of plant growth. 



Twenty years ago the work on plant growth substances was the 

 preoccupation of a few botanists in European laboratories, who dealt 

 with fractions of a milligram of an active material, not yet chemically 

 defined and present in plants in concentrations too small to be detected 

 except through its physiological activity. In this country the first small 

 laboratory for plant growth substance research was being constructed 

 in Pasadena by Herman Dolk, as a result of T. H, Morgan's vision and 

 interest in the subject. From this slow beginning, the work has developed 

 with increasing rapidity, so that it now influences all branches of botany 

 and has had far-reaching agricultural applications. 



Today several thousand persons are engaged in investigations, manu- 

 facture, and applied work on growth-regulatory compounds with physio- 

 logical properties resembling those of the material originally obtained 

 from the oat seedling. In this country synthetic plant growth regulators 

 have become one of two main groups of organic chemicals for agricultural 

 use. For the purpose of weed control alone nearly twenty million acres 

 of crops were treated with these compounds during the past year. 



A rapid stream of new information pertaining to the physiology, 

 biochemistry, chemistry, and agricultural uses of growth substances is 

 now pouring out from laboratories all over the world. To gain a per- 

 spective of the present status and progress in the field as a whole, a 



