6 Plant Life and Evolution 



plasm. What were the sources of energy for these 

 assimilation processes, of course we cannot tell, 

 since the requisite energy may have been of various 

 kinds and derived from various sources, light, 

 heat, or chemical energy, as any of these forms of 

 energy might very well serve the same purpose. 

 Thus the assimilation of CO 2 by the green plants 

 requires light for its accomplishment, but the nitro- 

 gen bacteria are able to assimilate CO 2 in the ab- 

 sence of light, probably through some form of 

 chemical energy. 



When life first appeared upon the earth the tem- 

 perature was presumably very much higher than at 

 present, and chemical activity would probably have 

 been more active. It has been supposed also that 

 CO 2 was more abundant in the atmosphere than at 

 the present time. Inasmuch as the assimilation of 

 CO 2 by the nitrogen bacteria is independent of light, 

 it is not necessary to assume that this assimilation 

 in the earliest forms of life was photosynthetic ; 

 but what agency transformed the presumably highly 

 unstable and complex inorganic compounds, which 

 antedated the first living things, into living proto- 

 plasm with its power of assimilation and growth, 

 we do not know. It may be that further experi- 

 mental work on these simple living things, the 

 nitrogen bacteria, may furnish the key to the mys- 

 tery of the origin of life. 



The distinguished botanist, Pfeffer, says : " It re- 

 mains uncertain whether the conditions now exist- 



