Daniel I. Anion 339 



This it not to say that zinc is not the metal specifically concerned in the 

 photochemical reactions of photosynthesis, but to suggest that such a 

 conclusion is as yet not supported by incontrovertible evidence. It is 

 hoped that experiments now in progress may throw some more light 

 on the subject. 



The foregoing discussion has attempted to demonstrate that an inte- 

 grated concept of the essentiality of mineral elements rests on the com- 

 bined contributions from studies of inorganic requirements for growth 

 and investigations on functional aspects of the essential nutrients. The 

 two approaches, which for the sake of convenience may be designated 

 as the physiological and biochemical, respectively, are mutually supple- 

 mentary. It was shown how in the case of chloride, its proposed essen- 

 tial status as a coenzyme for photosynthesis could not be accepted in 

 the light of evidence from growth experiments. It is possible, however, 

 that in the future an insight into the essential function of an inorganic 

 element may be gained from biochemical studies, well in advance of 

 any knowledge gained from growth experiments. A case in point is 

 cobalt in animal nutrition. Cobalt was recognized as a micronutrient 

 essential for ruminants (j6), but there was no evidence that it was re- 

 quired by other animals. Recently a striking discovery was made that 

 cobalt is a component of vitamin B 12 , which is identified with the "anti- 

 pernicious anemia" and the "animal-protein" factors (42,43,41). Vita- 

 min B 12 seems to be essential for all animals, and cobalt, which was 

 not previously found in a compound of a natural origin, must therefore 

 now also be regarded as an essential micronutrient for animals other 

 than ruminants. (However, recent experiments indicate that in rumi- 

 nants at least, cobalt may have functions distinct from its association 

 with vitamin B 12 ; Becker et al„ /j). Evidently the quantitative re- 

 quirement for cobalt by nonruminants is so small that it escaped de- 

 tection in direct growth experiments. An idea as to the experimental 

 difficulties which may be involved is given by the fact that four tons of 

 liver were required to yield one gram of the pure vitamin (42). 



The study of the inorganic requirements of higher plants has, apart 

 from its richly rewarding scientific purpose in contributing to the un- 

 derstanding of plant growth and metabolism, an important practical 

 objective as well. Developments in this field have provided in the past, 



