PHOTOSYNTHESIS 455 



ability of inorganic sources of energy. However, when condi- 

 tions on the surface of the Earth became oxidising these 

 sources were rather quickly exhausted and were only re- 

 plenished comparatively slowly from the deeper layers of the 

 crust of the Earth. On the other hand, the balance of organic 

 substances in the biosphere became more and more positive 

 owing to the appearance and quick development of photo- 

 autotrophs. Nowadays, although chemoautotrophs play an 

 important part in the circulation of sulphur, nitrogen, etc., 

 they have long ago been relegated to a secondary position 

 as producers of organic substances by green plants, and only 

 constitute a fraction of one per cent, of the general mass of 

 living things. 



Photosynthesis . 



Undoubtedly the highway of autotrophic development was 

 photosynthesis as we see it now in gi^een plants/" The use 

 of water as a hydrogen donor in photosynthetic organisms 

 was a tremendous advance in the development of biochemical 

 systems which linked the light-induced stage of the process 

 with the cycles of reactions and which brought about the 

 gradual reduction of CO2 and the formation of molecular 

 oxygen. 



However, the taking of this step required the prolonged 

 evolution of organisms which were already rather highly 

 developed and which possessed a large arsenal of diverse 

 metabolic mechanisms. Our knowledge of the photosynthetic 

 apparatus of contemporary plants convinces us that this must 

 be so. It is extremely complicated and, in spite of much 

 research, it is still far from being fully worked out. 



In order to give a general picture of what happened, we 

 may make the following analogy although, of course, it must 

 not be pushed too far. We may take a motor-car engine 

 as our example of a complicated system which carries out a 

 particular job. The work of the engine does not depend 

 exclusively on its essential component, the cylinder block. 

 It also depends on a number of accessory mechanisms, some 

 of which are themselves complex, each with its own specific 

 task, e.g. the preparation and delivery of the combustible 



