CHEMICAL PHENOMENA IN LIFE 



that is known about morphological reactions must 

 be assigned to these reaction-phenomena. 



Biologists are nowadays inclined to explain the 

 phenomena of adaptation in plants and animals 

 by the supposition that the hereditary adapted 

 forms took their origin from transitory morphoses, 

 which often do not last longer than the time during 

 which the external stimulus is acting on the 

 organism. In such a way may for instance be 

 understood the origin of dorsiventrality in plants. 

 A branch of ivy develops its rootlets only on the 

 shade-side, and turns its leaves all to the sun-side. 

 If we turn the branch by 180 degrees and fix it 

 in this new position, it changes its morphological 

 properties entirely, corresponding to the new 

 conditions. The old rootlets shrink and fall, but 

 new climbing roots are formed on the side which 

 is now turned away from the light. The dorsi- 

 ventrality is, as we see, not fixed. A branch of a 

 pine tree when turned by 180 degrees behaves 

 quite differently. The old part does not change 

 its character, and in spite of the unnatural position 

 the leaves remain without any reaction. But 

 when in the following spring the branch continues 

 its growth, the new part of the branch corresponds 

 in its formation exactly to the new position. We 

 see that a reaction could not be carried out in the 

 adult part of the branch, but the characteristics of 

 this part were not transferred to the new part. 

 The latter behaves according to its real life con- 

 142 



