68 



of plants is a question that the plant physiologist does not seri- 

 ously entertain. 



In selecting for discussion present-day problems which may be 

 considered fundamental, one is embarrassed by the wealth of 

 material and therefore but one more or less connected series 

 of topics which leads up to the modern mechanistic conception 

 of life processes has been chosen. In doing so it has been nec- 

 essary to ignore equally important questions which, though de- 

 veloped 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 neces- 

 sary 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 recognize, 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 pro- 

 duced, — a substance 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 outside 

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

 elaboration of this first substance formed that the living proto- 

 plasm is apparently necessary. At any rate we know that the 

 energy demanded for the process must be afforded by the par- 

 ticular rays of sunlight which the chlorophyll absorbs. 



In this photosynthetic activity of the green plant the carbohy- 

 drate supply of the world has been accounted for, but there is 

 an equally important question not concerned in this process, 

 namely, the source of nitrogen. Nitrogen is of course an essen- 

 tial element for the construction of protoplasm. As is well 

 known most plants can utilize it in simple combination with oxy- 

 gen in the form of a nitrate, a sharp contrast, by the way, to the 

 typical animal which requires it offered as an organic compound. 

 It is also known that the same plants cannot assimilate the free 

 nitrogen of the atmosphere, and further, in the processes of decay, 

 free nitrogen is liberated by the breaking down of the nitrogen 

 compounds in dead organic matter. The logical conclusion of 



