412 ON SUPPOSED BOTANICAL PROOFS OF [VII. 



The same argument also holds with regard to heliotropism. 

 The power of growing towards the light possessed by green 

 shoots cannot be a primitive character of the plant : it must 

 have arisen secondarily. If it were an essential and original 

 character it could not be reversed in certain parts of the plant ; 

 but the roots are negatively heliotropic, for the}'- grow awa}^ 

 from the light. There are also shoots, such as the climbing 

 shoots of Ivy, which are similarly negatively heliotropic. 

 Whenever the heliotropic power is thus reversed in shoots, the 

 change is of a useful kind. Thus the shoots of the Ivy gain the 

 power of clinging closely to a perpendicular wall or to some 

 horizontal plane ^. In this case, however, it is only the shoot 

 which is negatively heliotropic, its leaves turn towards the light ; 

 and the same is true of the flower-bearing shoots which do not 

 climb. All these are clearly adaptations and not the results of 

 direct influence. The light only provides the stimulus which 

 calls forth the characteristic reaction from each part of the 

 plant, but the cause of each peculiar reaction lies in the specific 

 nature of the part itself which has not been produced by light, 

 but as we believe by processes of natural selection. If this 

 explanation does not account for the facts we may as well 

 abandon all attempts at understanding the useful arrangements 

 in organisms. 



Sachs has used the term anisotropism to express the fact that 

 the various organs of a plant assume the most diverse directions 

 of growth under the influence of the same forces. He also 

 states that anisotropism is one of the most general character- 

 istics of vegetable organization, and that it is quite impossible 

 to form any idea as to how plants would appear or how they 

 could live if their different organs were not anisotropic. Since 

 anisotropism is nothing more than the expression of different 

 kinds of susceptibility to the action of gravity, light, etc., it is 

 obvious that the configuration of the plant is to be traced to such 

 specific susceptibilities. 



Now these specific susceptibilities cannot have been produced 

 by the direct effect of the various external influences (as was 

 shown above), and the only other possible explanation is to 

 recognise them as adaptations, and to admit that they have 



^ Compare Sachs, ' Lectures on the Physiology of Plants/ translated 

 by H. Marshall Ward, p. 710. 



