58o PLANT RESPONSE 



been found difficult to resolve a given movement into its 

 components, and this has led to the abandonment of the 

 attempt to relate these various movements to any single 

 basic reaction. So far was this carried that, in spite of the 

 well-known observation that a radial organ, illuminated with 

 different intensities on two different sides simultaneously, 

 bends in the direction of the more intense illumination, 

 Sachs found himself compelled to believe that it was the 

 direction and not the intensity of light that determined the 

 responsive movement. 



Theory of de Candolle. — It is appropriate to make here 

 a brief mention of the theory of de Candolle, which has 

 hitherto met with unmerited neglect. De Candolle started 

 froin the known fact that light retards growth, and explained 

 growth-curvature as due to the relatively greater growth of 

 the shaded, inducing concavity of the lighted, side. This 

 explanation of the mechanics of such movements constitutes 

 an important advance, though it does not take full account 

 of all the factors of the problem. This theory of de Candolle 

 has, however, been discarded, in consequence of the difficulty 

 which it presented of explaining the action of the negatively 

 heliotropic organs, in which the lighted side is found to be 

 convex. Extending this theory to cases of negative helio- 

 tropism, it was regarded as a logical inference that light 

 should here accelerate growth. It is doubtful, however, 

 whether such an inference is justifiable. In any case this 

 was negatived by the researches of Miiller-Thurgau, F. 

 Darwin, and Wiesner, who showed that light retarded general 

 growth in negative as well as in positive heliotropic organs. 

 It will be shown, however, in the course of succeeding 

 chapters, that though the circumstances which modify the 

 response of the plant, so that it is exhibited as negative 

 heliotropism, are somewhat complicated, yet they in no way 

 detract from the theory of de Candolle as applied to positive 

 heliotropism. 



Inadequacy of de Candolle's theory. — The flaw in his 

 theory lies rather in the fact that he regarded the normal 



