is characteristic of the higher animal forms. While plants 

 can and do respond to differences in light intensity less than 

 that which the human eye can perceive, it is gratuitous to 

 suppose that there is anything analogous in the two pro- 

 cesses. The possibility of any reasoning action or instinct 

 on the part of plants is a question that the plant physiolo- 

 gist does not seriously entertain. 



In selecting for discussion present day problems whifti 

 may be considered fundamental, one is embarrassed by the 

 wealth of material and therefore but one more or less con- 

 nected series of topics which leads up to the modern 

 mechanistic conception of life processes has been chosen. 

 In doing so it has been necessary to ignore equally import- 

 ant questions which, though developed from no less a 

 mechanistic standpoint, are more scattered. 



In referring to the assimilation of carbon dioxide by 

 green plants and the production of organic food thereby, it 

 was necessary to admit that the details of the process are 

 not satisfactorily known. It is evident, however, that the 

 starch, which is the first substance that we readily recog- 

 nize, is not the first substance which is formed. Modern 

 research points more and more to the conclusion that it is 

 the simplest of carbohydrates that is produced, a sub- 

 stance known as formaldehyde. But what is especially 

 interesting is that it seems not impossible that this primal 

 reaction may not after all be a function of the living pro- 

 toplasm, but a chemical reaction that can be carried on out- 

 side the cell through the agency of chlorophyll. It is in 

 the further elaboration of this first substance formed that 

 the living protoplasm is apparently necessary. At any 

 rate we know that the energy demanded for the process 

 must be afforded by the particular rays of sunlight which 

 the chlorophyll absorbs. 



In this photosynthetic activity of the green plant the car- 

 bohydrate supply of the world has been accounted for, but 



19 



BIO-AGRICULTURAL LIBRARY 

 UNIVERSITY OF CALIFORNIA 

 RIVERSIDE, CALIFORNIA 92502 



