TROPISMS 153 



interfere with its passage. The transmission of the stimulus 

 is therefore due to a diffusible substance. Purdy (1921) 

 confirmed his essential results. 



Soon after this first important step Paal's investigations 

 (1914, 1919) brought another great advance. Paal found 

 that not only after tropistic stimulation, but also in normal 

 growth, the tip exerts an influence on the growth of the 

 lower zones. Thus he developed the concept that the tip 

 continuously forms growth-regulating substances. His great 

 contribution to the field of tropisms was that he envisaged 

 the response as due to an unequal supply of these growth 

 regulators to the two sides (c/. II C). Considering photo- 

 tropism in particular, he suggested three possible ways in 

 which such an effect might be brought about : the regulator 

 might ''disappear," its production might be reduced, or its 

 transport might be interfered with, on the lighted side. 



The concepts of Stark (1921), Stark and Drechsel (1922), 

 and Brauner (1922) represented a modification of Paal's 

 theories, tinged with a return to earlier ideas. Stark showed 

 that the traumatotropic stimulus, or the tendency of coleop- 

 tiles to curve towards a wound, could be transmitted, just 

 as in phototropism, across a cut surface. Tips wounded 

 on one side, e.g. with AgNOs, were replaced on stumps and 

 induced curvatures, in the stump, towards the wounded 

 ' side. If tissues were crushed and mixed with agar, then the 

 agar, if applied one-sidedly to stumps, caused curvatures 

 towards that side. Hence he assumed that there are special 

 wound-hormones, which inhibit growth. Similarly he as- 

 sumed special phototropo-hormones, (active in phototropism), 

 which increase the growth rate (cf. footnote to p. 114). 

 His evidence also indicated that these tropo-hormones had 

 definite specificity. 



Brauner (1922) placed unilaterally-illuminated tips on 

 unilluminated stumps, and unilluminated tips on unilaterally- 

 illuminated bases; in both cases a curvature resulted, while 

 the illumination of the stump alone caused no response. 

 He explained these results in terms of an increased permeabil- 



