160 



and form only a subordinate part in the general frame of his work. 

 His experimental objects were seedlings of Setaria and roots of 

 Sinapis. 



The choice of the first-named object is not very happy, as the 

 direct effect of (he exposition of the coleoptile is not ontwardly 

 visible, and its existence therefore, as yet purely hypothetical. The 

 indirect effect consists in an antitropic curvature of the axis. This 

 curvature which appears almost immediately, is followed in about 

 25 nnniites by a normal one. The latter should be tlie result of 

 the propagation of the invisible direct effect. An illumination of 

 the growing region gives a normal curvature. 



That the antitropic curvature of the axis occurring with an 

 exposition of the coleoptile should be the indirect effect of this 

 illumination in the sense of Bose, is possible. It should be remarked 

 however, that it is not proved. As yet, we don't know with 

 certainty, if in this case the direct effect consists really in a 

 contraction, as no sign thereof becomes outwardly visible. 



The roots of Sinapis show a negative phototropism. At least this 

 is the case, when both the tip and the growing region are exposed 

 to the light. The curvature appears in the growing region, the tip 

 always remaining straight. An exposition of the tip also gives a 

 negative curvature of the growing region, but if this part itself is 

 exposed to the light, there appears at first a positive curvature 

 which only after some time is followed by a weak negative one. 



Bosk considers the negative curvature in the growing region pro- 

 duced by an exposition of the tip, as the indirect effect, the direct 

 effect as in Setaria remaining concealed. That this curvature is not, 

 as in Setaria, followed by a positive one, he explains by assuming 

 that the intervening tissue would be practically unable to conduct 

 the direct effect. In the case of an exposition of the growing region, 

 the positive curvature is considered as the direct effect, whereas the 

 negative curvature appearing a little later, is said to arise on account 

 of transverse conduction of the direct effect under continued 

 illumination. 



However, this explanation is not very convincing. That a neutra- 

 lisation of the curvature might come about by transverse conduction, 

 is quite conceivable, but how a reversion of the curvature might 

 be explained in this way, 1 fail to understand. Moreover, as a 

 conductivity for the direct effect in (he longitudinal direction is 

 supposed to be absent, it is not readily admissible that is should be 

 very efficacious in the transverse direction. Therefore, in this case 

 the interpretation of Bose cannot be considered as sufficiently founded. 



