PHOTOCHEMICAL REACTIONS 447 



system, this scheme will still retain its significance. It demon- 

 strates very clearly ^vhat is the essentially new factor in the 

 development of photosynthesis in the form in which we know 

 it to-day in green plants. What is new is mainly concerned 

 with the giving off of molecular oxygen into the surrounding 

 atmosphere. 



The more primitive pigmented organisms had chemical 

 mechanisms which allowed them to use, as the primary 

 hydrogen donors in photosynthetic reactions, only the most 

 readily available or ' active ' donors such as, for example, 

 organic compounds, or such inorganic substances as hydrogen 

 sulphide or molecular hydrogen. As examples of organisms 

 which have retained this relatively simple photosynthetic 

 organisation we may take the purple and green bacteria 

 which were mentioned above. 



However, during the progressive evolution of the earliest 

 photosynthetic organisms their internal organisation became 

 both more closely knit and more elaborate, tending towards 

 the creation of mechanisms enabling them to use wider and 

 wider selections of substances as hydrogen donors. This 

 course of development inevitably led to the inclusion in the 

 photosynthetic reaction of the more ' difficult ', but also more 

 ubiquitous hydrogen donor, water. The oxygen of the water 

 was then liberated in molecular form. 



Some contemporary organisms are interesting in that their 

 metabolism retains features of a more primitive organisation 

 of the photosynthetic processes, though the ability to give off 

 the molecular oxygen of water is already manifest in them. 

 They seem to be intermediate links in the chain between 

 the earliest photosynthetic organisms and the highly organised 

 photoautotrophs. 



An example of an organism of this sort is the green alga 

 Scenedesmus, the metabolism of which has been studied in 

 detail from this point of view by H. Gaffron."^ Under 

 normal conditions this alga, like all other green plants, 

 carries out photosynthesis accompanied by the giving off of 

 oxygen. However, if it is kept for an hour or more under 

 anaerobic conditions and then placed under relatively weak 

 illumination in an atmosphere of hydrogen or nitrogen with 

 cOo, its metabolism will be substantially changed. Under 



