THE MOVEMENTS AND SENSITIVITY OF PLANTS 319 



as follows: a unilateral exposure to light causes certain inhibitory- 

 substances to appear in the organ of perception, which descend 

 to the growing zone and produce a delay of growth on the exposed 

 side. 



Recent experiments have shown that these inhibitors are 

 definite substances and that the transmission of excitation from the 

 zone of perception to the zone of reaction proceeds by diffusion. 

 Boysen-Jensen's experiments may be considered as a definite proof 

 of this fact. Though his results were at first seriously doubted, 

 later they were fully confirmed by a number of other investiga- 

 tors (Paal, Stark, Wendt, Cholodny, and others). These experi- 

 ments have shown that excitation caused by a unilateral exposure 

 to light of the tips of shoots of oats which possess a peculiarly high 

 sensitivity, and therefore usually serve as objects for phototropic 

 experiments, is transmitted downwards to the zone of reaction not 

 only through the living tissue, but through a thin layer of gelatin 

 as well. The tip of the shoot serving for this experiment is first 

 cut off and then fixed in its former position by means of gelatin. 

 Further experiments have shown that the tip may not only be 

 attached to the same shoot from which it was removed, but to 

 another previously decapitated shoot as well, and that this is also 

 followed by the formation of a curvature even in the case when the 

 shoot to which a foreign "head" has been affixed was not subjected 

 to unilateral exposure to light (Fig. 129). Moreover, it is possible 

 to attach the removed tips of oat shoots to the decapitated shoots 

 of other cereals, such as wheat, barley, and rye, and still the exci- 

 tation produced in the tip will cause the formation of a curvature, 

 though a somewhat less marked one, in the lower parts of the shoot 

 which are alien to them. Even the root tip of maize stuck to the 

 coleoptile of oats, having lost its phototropic sensitivity as a result 

 of decapitation, enables it to react on unilateral illumination. 



Excitation, however, is not transmitted through a layer of 

 mica or tinfoil. This leads us to believe that we have to deal with 

 the phenomenon of diffusion and not with the transmission of an 

 electric charge. 



If all the data concerning the physical nature and the manner 

 of the transmission of phototropic stimuli are correlated with the 

 dependence of the rate of growth (Art. 86) on the amount of the 

 growth hormone manufactured by the stem tip or the coleoptile, 

 the following mechanism of phototropic curvatures (Cholodny, 



