114 PHOTO-SYNTHESIS IN FRESH- WATER 



long intermediate range of evolution. The first synthesising system 

 acting upon the light was hence probably an inorganic colloidal 

 system in solution, capable of adsorbing the simple organic substances 

 which it synthesised. It is hence futile to search for the origin of 

 life at the level of bacteria and torulae. 



5. As complexity increased with progressive evolution more and 

 more rapid transformers for the capture of the energy of the sunlight 

 came into existence. Such transformers are found in the green 

 cell for fixation of both carbon and nitrogen. The earlier trans- 

 formers in the inorganic colloidal systems can only utilise light of 

 short wave-lengths; the later transformers in the living cells are 

 adapted to utilise longer wave-lengths, and the very short wave- 

 lengths, which are lethal, are cut off by their colour screens of 

 chlorophyll, etc. 



6. The earliest products of photo- synthesis, such as formaldehyde 

 and methylic alcohol, are highly poisonous to the green cell; but 

 fed to it at sufficiently high dilution, can be used as nutrition in 

 absence of carbon dioxide, and very marked growths have been 

 obtained with these substances as the sole source of carbon. 



7. In the absence of all other sources of nitrogen save the 

 elemental nitrogen of the atmosphere, but with abundance of carbon 

 dioxide, the unicellular algae can fix nitrogen, grow and form 

 proteins. 



8. The rate of fixation and growth is, however, greatly accelerated 

 if nitrites or oxides of nitrogen are available. 



9. These oxides of nitrogen can be supplied in gaseous form from 

 the atmosphere, and pure country air normally contains such oxides 

 of nitrogen, especially in spring and summer. 



