F. W. WENT 71 



This means that Boysen Jensen definitely thought in terms of diflfusible 

 materials, either substances or ions, hut that the complexity of the 

 prevailing views on stimuli in plants prevented him from assuming any 

 connection between such substances or ions and simple growth. Not 

 only Boysen Jensen failed to see this connection, but so did all his 

 contemporaries, including Pfeffer. It seems that on the whole more 

 significance was attributed to Fitting's experiments, which were pub- 

 lished in much greater detail, and the decapitation experiment was 

 considered a curiosity. Many investigators tried to explain it away, 

 pointing out that since decapitation involved very serious wounding, 

 the transmission might, therefore, be simulated by wound reactions. 



Pfeffer was also much worried by Boysen Jensen's experiment, and 

 he induced another visitor to his laboratory, A. Paal from Budapest, to 

 repeat it. This Paal (16,17) ^^^ i^ great detail, varying the experiment 

 in every conceivable way. And even a modern statistician would have 

 been satisfied with the total numbers of plants used. The most important 

 thing, however, was that Paal probed deeply into the nature of the 

 transmission and found that it was just a case of unequal distribution of 

 a growth-promoting substance, which is formed in stem tips all the time. 

 This liberated plant physiology from some of the mysticism connected 

 with tropisms and the stimulus concept. It opened the way to many new 

 and interesting experiments, and it was the first generally accepted 

 demonstration of the existence of a correlation carrier In plants. 



We come now to the second stage In the development of our knowledge 

 about plant hormones. This really started with the work of Paal, who 

 clearly stated that in the normal coleoptile tip a growth-promoting 

 substance was formed continuously, which regulated the growth of the 

 cells below the tip. This, and not Boysen Jensen's work, broke the 

 spell cast by Fitting and so many others on the problem of the trans- 

 mission of the phototropic stimulus. Many investigators, such as Stark, 

 Drechsel, Seubert, Soding, Brauner, and Nielsen, started to work on 

 growth-regulating substances In stems and coleoptlles, all within a few 

 years from the publishing of Paal's paper. Everything pointed towards a 

 speedy solution of this problem. 



Since so much of the basic knowledge about tropisms in plants had 

 come from my father's laboratory in Utrecht, and since at the time the 

 growth-regulating substance seemed to be particularly important in the 

 understanding of tropisms, it is obvious that this substance was much 



