6 GROWTH HORMONES IN PLANTS 



of Avena saliva, in the hypocotyls of Brassica oleracea and Beta 

 vulgaris, and in the negatively phototropic movements of roots. 



Wiesner. — Although the significance of Darwin's investigations 

 was recognized fully in the first edition of Pfeffer's "Plant 

 Physiology," the conclusions were criticized adversely by 

 Wiesner (1881). 



This criticism is of only historical interest, but it should be 

 mentioned briefly. The curvature that occurs in the lower part 

 of the hypocotyl of Brassica oleracea when the tip is illuminated 

 was explained by a factor that Wiesner called a "traction growth " 

 {Zugwachstum) . When the upper part curved under the influ- 

 ence of unilateral light, its weight supposedly had a unilateral 

 effect upon the lower part. The extended shaded side was 

 interpreted as growing more rapidly than the compressed front 

 side. 



This point of view does not seem at all convincing; moreover, 

 only a few experiments were performed with grass coleoptiles. 

 Yet, doubt was thrown upon the correctness of Darwin's con- 

 clusions. At the suggestion of Pfeffer, Rothert (1894) investi- 

 gated the problem of phototropic stimulus conduction in a very 

 thorough manner at the Leipzig laboratory. 



Rothert. — The outcome of Rothert's work was a complete 

 confirmation of that of Darwin. Conduction of the phototropic 

 stimulus was demonstrated in a series of different plant organs, 

 including coleoptiles of grasses, seedling axes of numerous 

 dicotyledons, orthotropic leaves, petioles, etc. It developed, 

 however, that the localization of phototropic sensitivity was not 

 actually so marked as had been supposed from Darwin's experi- 

 ments. Darwun had maintained that only the tip of the Avena 

 coleoptile is phototropically sensitive; Rothert showed that a 

 weak phototropic curvature resulted from unilateral illumination 

 of the basal part. It was found also that sensitivity and mobility 

 are very sharply separated in the seedlings of some grasses. In 

 certain cases only the coleoptile was sensitive to light, and the 

 curvature took place in the first internode below the coleoptile 

 where sensitivity was completely lacking. 



With regard to the paths of stimulus conduction in the Avena 

 coleoptile, Rothert noted the following: In the coleoptile cylinder 

 two vascular bundles are situated opposite one another, not 

 joined by cross-connecting bundles (Fig. 21). In an attempt to 



