324 VITAMIN REQUIREMENTS 



This chapter, then, includes substances, primarily the water-soluble 

 vitamins, which are thought to be catalytic in function; the amino 

 acids and nitrogen bases are considered in Chapter 8, as is the plant 

 growth factor indoleacetic acid. 



The history of our knowledge of the vitamin requirements of fungi 

 is reviewed by Janke (117), Robbins and V. Kavanagh (213), Schopfer 

 (239), and Knight (122). Although the work on yeasts and yeastlike 

 fungi really began in the nineteenth century, the vitamin needs of the 

 filamentous fungi have been studied intensively only since 1934, i.e., 

 since the papers of Schopfer (236, 237) on thiamine and the growth of 

 Phycomyces blakesleeanus. Most work has been devoted to the require- 

 ments of naturally occurring fungi and, more recently, to the require- 

 ments of induced mutants; both types of material have provided signif- 

 icant information on the biosynthesis and the function of vitamins in 

 living things generally. 



1. PROBLEMS OF METHOD 



The central methodological problem in vitamin studies is that of the 

 degree to which the investigator knows the composition of the medium. 

 Almost any natural product, e.g., sucrose or asparagine, is contaminated 

 with significant amounts of some vitamins; procedures for purification 

 will be found in the literature on particular growth factors. 



For very limited purposes, e.g., in formulating a medium for routine 

 use, it may be sufficient to use the approximate methods so common in 

 the literature on vitamin nutrition. However, a serious study of vita- 

 min nutrition requires at the least that three methodological criteria be 

 met: 



1. The measurement of growth should be objective and quantitative. 

 Although growth on agar has been used fairly successfully in particular 

 problems, it cannot be relied upon, and dry weight or turbidity is to be 

 preferred (Chapter 1). Pyridine-extracted agar (214) may, however, 

 be used for some problems. 



2. Over some range of concentration of the vitamin, it should be 

 shown that growth is a function of dose (concentration or total amount 

 of the vitamin). 



3. If growth occurs on the basal medium lacking added vitamin, an 

 independent assay of the medium for the vitamin should be made. 

 One very simple test consists of growing on the basal medium an or- 

 ganism known to require the factor absolutely. Other problems of 

 method are summarized by Robbins and V. Kavanagh (213). 



