BIOGENESIS OF THE B VITAMINS 79 



but the available evidence indicates that the contribution is substantial. 

 To think of green plants as the sole substantial producers of thiamine and 

 other B vitamins is certainly unwarranted. 



Second, we can be sure that thiamine is produced in nature by yeasts 

 and molds. Brewers' yeast was one of the earliest discovered rich sources 

 of "vitamin B," and that the source of thiamine in yeasts is not entirely 

 in the culture medium has been amply demonstrated. 2,6 While some 

 strains of the yeast Saccharomyces cerevisiae respond by enhanced growth 

 to thiamine, 7 many do not, and at least some of those which do respond 

 are capable of thiamine synthesis. 2, 8 It is interesting that yeasts, when 

 furnished with thiazole and pyrimidine intermediates used in the chemical 

 manufacture of thiamine, may produce relatively large amounts of thi- 

 amine. This fact has found commercial application. 9 



It is presumed that thiamine synthesis takes place in many molds 

 because culture media for them have since Raulin's time been made up 

 from constituents of known composition excluding all vitamins. More 

 definite evidence of synthesis has been found, for example, in the case 

 of Aspergillus oryzae 10 and Aspergillus niger. 11 Schopfer and co-workers 

 have made extensive studies which have revealed that while some fungi 

 can synthesize thiamine using simple starting materials, others require 

 either the thiazole or pyrimidine portions of the molecule, or both, before 

 synthesis and growth are accomplished. 12 The vitamin requirements of 

 fungi have also been investigated to some extent by Williams and Honn 13 

 and by Robbins and Kavanagh. 14 



The importance of fungi as producers of thiamine and of other B vita- 

 mins in nature is indicated. While the number of mold organisms in the 

 soil is small compared with bacteria, the total mold substance present is 

 said to be larger. Furthermore, the close association of fungi with the roots 

 of many green plants is suggestive. 



The synthesis of thiamine in green plants during the course of their 

 natural growth is undoubtedly of prime importance in connection with 

 producing the thiamine supply of nature. This does not mean, however, 

 that one can glibly affirm that green plants are the responsible agents in 

 this synthesis. To demonstrate conclusively the synthetic production of 

 thiamine by a green plant, one would have to grow the plant aseptically 

 from seeds rendered aseptic, and show an increase in the thiamine content 

 of the system. While this is probably possible, it has rarely, if ever, been 

 done. 



In this connection it is necessary to call attention to the most unsatis- 

 factory state of our knowledge with respect to biochemical facts under- 

 lying the symbiosis which commonly exists between green plants and 

 mycorhizal fungi and the bacteria of the soil. Because of this lack of 



