12 PHYTOHORMONES 



enced by scattered light, by contact stimulus, or by the 

 asymmetrical weight of the bending tip, confirmed Boysen 

 Jensen's finding. Varying the conditions of the experiment, 

 he showed that the stimulus could cross a layer of gelatin, 

 but not cocoa-butter, mica, or platinum foil. His next im- 

 portant step was to show that, even without light, curva- 

 tures could be induced in the base by the simple process of 

 cutting off the tip and replacing it on one side of the stump. 

 This makes it clear that ''the tip is the seat of a growth- 

 regulating center. In it a substance (or a mixture) is formed 

 and internally secreted, and this substance, equally dis- 

 tributed over all sides, moves downwards through the living 

 tissue. In the growing zone it causes symmetrical growth. 

 If the movement of this correlation carrier is disturbed on 

 one side, a growth decrease on that, side results, giving rise 

 to a curvature of the organ" (Paal, 1919). This puts the 

 whole problem on a new basis, namely the control of normal 

 growth by a diffusible correlation carrier. Here, for the 

 first time, the idea of a groivth hormone enters botanical 

 literature. Paal then suggested that "this correlation car- 

 rier, which, under normal conditions continually moves 

 downwards from the tip along all sides, is, upon illumination 

 of the tip, either interfered with in its formation, photo- 

 chemically inactivated, or inhibited in its downward move- 

 ment, through some change in the protoplasm, these effects 

 being greater on the lighted side." 



Paal thus established the theory that the growth of the 

 coleoptile is controlled by the tip through the agency of a 

 diffusible substance, and this was confirmed by the careful 

 growth measurements of Soding (1923, 1925). Working not 

 with curvatures but with straight growth, Soding proved 

 that replacement of the cut tip would restore the greater 

 part of the growth reduction which is caused by decapita- 

 tion. Further, the success of Paal's work led to a search for 

 a direct demonstration of the postulated growth-promoting 

 substance. Stark (1921) introduced the method of apply- 

 ing small blocks of agar on one side of decapitated coleop- 



