8 GROWTH HORMONES IN PLANTS 



When two incisions were made one above the other, he thought 

 that the stimulus conduction was not disturbed by the insertion 

 of tinfoil, no matter how oriented with respect to the direction of 

 light. However, if there were two transverse incisions with tin- 

 foil or mica inserts one above the other, the phototropic curvature 

 under the incisions was very slight. Fitting surmised that the 

 absence of curvature in the latter case was due to drying out of 

 the tip, since the vascular bundles had been severed. 



The phototropic curvature in the Avena coleoptile is caused by 

 a difference in the rate of growth of the two sides, the darkened 

 side growing more rapidly than the illuminated. Fitting 

 observed that a positive phototropic curvature resulted whether 

 an incision was made upon the lighted (front) or upon the shaded 

 (back) side. This meant that if stimulus conduction occurred 

 only in living tissue, it must take place in the former case on the 

 back side and in the latter on the front side. The final result 

 of stimulus conduction is the same in both cases; i.e., the shaded 

 side of the Avena coleoptile grows more rapidly than the illumi- 

 nated. Fitting explained this with the assumption 



. . . that the polar opposition (polarity), which is induced by an exter- 

 nal stimulus in all parts (cells) of the organ of perception, is spread out 

 through the living tissues in a physiologically radially symmetrical zone 

 of response. There is no lateral polarity; all the parts (including all 

 the cells) which made up the path of stimulus conduction become 

 "polarized" (longitudinally) in the same way. Because of this the 

 responding zone gives rise to a curvature, either positive or negative, 

 which is determined solely by the direction of this polarity. The polar- 

 ity is dependent indirectly upon the external stimulus. Curvature 

 increases until this "polarity" is removed again, according to the 

 circumstances. 



Stated in other words: The individual cells of the unilaterally 

 illuminated tip become polarized so that a difference arises 

 between the front and the back side, and this polarity is trans- 

 mitted to the cells in the darkened basal portion. 



Boysen Jensen. — In 1907, before Fitting's work was published, 

 Boysen Jensen began experiments on the processes of stimulus 

 conduction in the Avena coleoptile. It was found that the con- 

 duction of a stimulus from the unilaterally illuminated tip to the 

 darkened basal portion could be arrested by an incision upon the 

 back side, while this was not the case when an incision was made 



