INTRODUCTION AND HISTORICAL SKETCH 15 



tropic curvature was demonstrated, therefore, through these 

 investigations. 



Contemporaneously with these studies, in the years 1909 and 

 1910, Fitting pubUshed two works in which it was shown that 

 orchid polUnia contain a substance that produces a swelhng of the 

 gynostemium. According to Fitting (1910), this substance is a 

 hormone; and according to the more recent investigations of 

 Laibach (1932) and of Laibach and Maschmann (1933), it is 

 probably identical with the growth substance of the Avena 

 coleoptile. 



In the past few years, marked advances have been made in our 

 knowledge of the occurrence, movement, and quantitative 

 determination of the plant-growth substances (Went, 1928a). 

 Recent outstanding contributions to the chemistry of the subject 

 (Kogl, Haagen Smit, and Erxleben, 1932-1935) have opened up 

 new phases of the general investigation which may become 

 valuable in horticultural practice (Bouillenne and Went, 1933; 

 Hitchcock and Zimmerman, 1935; Cooper, 1935). Detailed dis- 

 cussion of the more significant aspects of the growth-substance 

 problem will be presented in the chapters that follow. 



SUMMARY 



The starting point for growth-substance investigations was 

 the demonstration of a growth-promoting material in the tip of 

 the Avena coleoptile, as shown by phototropic curvature. The 

 brief historical sketch wliich has been presented here indicates 

 that the growth-substance explanation of photo- and geotropism 

 had its origin many years ago. About one-quarter of a century 

 has elapsed since a hypothesis was suggested according to which a 

 stimulus substance in the coleoptile was displaced by the effect of 

 unilateral light, or gravity (Boysen Jensen). Other contribu- 

 tions to the solution of the problem followed (Paal, Stark, and 

 Seubert). A new impetus was given to the subject when Went 

 (1927, 1928a) published his method of procedure for extracting 

 growth substance and demonstrating the quantitative relation- 

 ship between it and growth in the Avena coleoptile. An ever 

 increasing fund of knowledge about hormone activity is con- 

 tinually extending our understanding of tropisms and the whole 

 problem of normal growth. 



