n6 THE ORIGIN OF THE NERVOUS SYSTEM 



According to this conception the nervous system is 

 the physiological and morphological expression of the 

 excitation-transmission relations, first with respect to 

 the primary or chief physiological gradients, and later 

 with respect to the progressive developmental complica- 

 tions as they arise. 



The absence of a nervous system in plants apparently 

 depends upon the inability of plant protoplasm to 

 synthesize molecules sufficiently stable under the exist- 

 ing physiological conditions to constitute the basis of a 

 permanent morphological structure of nervous character, 

 that is, possessing a stable protoplasmic structural sub- 

 stratum and at the same time a mechanism permitting 

 intense dynamic activity. In general, the capacity of 

 plant protoplasm for morphological differentiation is 

 slight as compared with that of animal protoplasm, and 

 plant structure consists very largely of cellulose. Con- 

 sequently physiological integration in plants retains 

 more or less its primitive character, based on a gradient 

 in physiological activity, physiological dominance of 

 the apical region, the growing tip, is not very effective, 

 and repeated physiological isolation and reproduction, 

 as evidenced by the development and arrangement of 

 multiple axes in the form of branches and various other 

 parts, is a characteristic feature of the life of most plants. 



This conception of the ontogenetic origin of the 

 nervous system is purely physiological and involves no 

 assumptions concerning inheritance or evolution. The 

 gradients do not determine the nature of nervous struc- 

 ture but merely provide the physiological basis for the 

 realization of certain of the hereditary potentialities 

 of the protoplasm concerned and the problem of the 



