330 Mineral Nutrition of Plants 



medium possessing an optimum supply and favorable conditions for 

 the absorption of the essential elements cannot be improved by the 

 addition of nonessential elements, subject of course, to such reserva- 

 tions about possible unknown micronutrients as were discussed earlier. 



The discussion thus far has tended to show how, in many instances, 

 the study of function has strengthened or clarified the conclusions 

 derived from growth experiments about the indispensability of inor- 

 ganic nutrients. Recent studies in our laboratory have also shown, 

 however, that conclusions drawn from growth experiments can greatly 

 aid in the interpretation of biochemical observations on function. Be- 

 cause of the pertinence that these results (12) have to the problems 

 under discussion, it is proposed to relate them in some detail. 



It should be stated at the outset that the objective of the investigation 

 which yielded the results to be examined was somewhat different from 

 the topic under discussion. For the past several years we have been 

 interested in exploring the possible function that inorganic elements, 

 already recognized as essential for plant growth, may have in photo- 

 synthesis. We were encouraged to embark on this investigation by the 

 important discovery of Hill (20) that the long-known capacity of 

 isolated chloroplasts to evolve oxygen can be greatly enhanced by the 

 use of suitable oxidants. Here was a subcellular system which retained 

 the ability to carry out in vitro the photochemical reaction peculiar to 

 the photosynthesis of green plants: the evolution of oxygen resulting 

 from the splitting of water through the capture of the energy of light. 

 The evidence in favor of the identity of the oxygen-liberating mechan- 

 ism in isolated chloroplasts with that in the intact green cells has 

 recently been reviewed by Holt and French (24) and further elaborated 

 by Arnon and Whatley (//). 



The photochemical evolution of oxygen by chloroplasts isolated from 

 sugar beet and spinach was recently investigated by Warburg and 

 Liittgens (52) who reached the rather striking conclusion that the 

 chloride ion was a coenzyme essential for the photochemical reactions 

 in photosynthesis. That such a simple yet important fact escaped the 

 notice of all the workers in this field was indeed cause enough for 

 Warburg and Liittgens (52) to remark how rash were all the previous 

 theories on the mechanism of photosynthesis. The evidence which led 



