VIII RECRESCENCE 389 



which extraordinarily minute quantities of material often 

 cause the reaction. Sensitiveness of this kind in relation to 

 certain stimuli is inherited. There can, however, be no doubt 

 that light and air have had an influence in the manner supposed 

 by me also on the permanent inherited position of leaves. 



Indeed, I believe that leaves in general owe their origin 

 partly to the action of light and air. That adaptation has 

 had some influence in their production, both generally and in 

 particular in the evolution of their more minute structure, is 

 to me self-evident, but that, e.g. the variety of form of the 

 leaves of our native trees is essentially due to adaptation, is a 

 belief for which I can iind no basis. I consider the differ- 

 ences rather to be for the most part the simple results of 

 growth as affected by direct external stimuli, principally by 

 nourishment, a conclusion supported by the changes which 

 occur when the nutrition is altered. Of course the forms are 

 partly determined by the veins of the leaves, which are the 

 channels for the passage to and fro of the nutritive material, 

 and which have the additional function of keeping the leaves 

 expanded. But it is not possible to say that just this or 

 that distribution of the veins is a necessary requirement of 

 adaptation — this also is an obvious consequence of definite 

 directions of crrowth. 



The Eecrescence of Lost Parts as an Example of 

 Organic Growth 



The discussion of the influence of definite directions of 

 growth on the form of organisms leads me to the subject of 

 recrescence, which it seems to me can only be explained by 

 the aid of my theory of organic growth. 



Eecrescence is, in fact, nothing but an efi'ect of the same 

 causes wdiich condition growth in definite directions. Among 

 cases of recrescence two kinds can be distinguished (1) 



