CHAPTER VII 

 STUDIES OF PHOTO-SYNTHESIS IN FRESH-WATER ALG^I 



1. THE FIXATION OF BOTH CARBON AND NITROGEN FROM THE ATMO- 

 SPHERE TO FORM ORGANIC TISSUE BY THE GREEN PLANT CELL. 

 2. NUTRITION AND GROWTH PRODUCED BY HIGH GASEOUS 

 DILUTIONS OF SIMPLE ORGANIC COMPOUNDS, SUCH AS FORMAL- 

 DEHYDE AND METHYLIC ALCOHOL. 3. NUTRITION AND GROWTH 

 BY MEANS OF HIGH DILUTIONS OF CARBON DIOXIDE AND OXIDES 

 OF NITROGEN WITHOUT ACCESS TO ATMOSPHERE. 



THE two most primeval and most fundamental chemical processes 

 for living organisms are those two in which their living substance is 

 synthesised from inorganic sources with uptake of energy. By one 

 of these carbon is built into organic compounds, starting with the 

 oxidised carbon dioxide of the atmosphere and utilising the energy of 

 sunlight. The source of the inorganic nitrogen, which in the second 

 is likewise built into organic forms in the amino- acids and proteins, 

 is more obscure, and has in the course of 150 years led to much 

 disputation. In the present and succeeding chapters evidence will 

 be adduced that the source of the nitrogen utilised by the plant does 

 not lie in the soil (although a luxury or luxus supply may be given 

 from the soil), but in the air, and that the reaction by which the 

 atmospheric nitrogen and oxygen are made reactive is a photo- 

 synthetic one, in which the energy of sunlight is absorbed and 

 converted into chemical form as nitrites in the green cell. 



This view places these two processes of carbon and nitrogen 

 assimilation upon the same basis, and makes them coeval in the 

 process of evolution; this, as will presently be pointed out, must 

 have been the case, in order that any living organism could ever 

 have appeared upon the earth. 



The importance of this question of the fixation of nitrogen has 

 induced much study of it and led to many polemics, and it is in- 

 teresting how invariably the weight of scientific authority has been- 



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