NUTRITIONAL REQUIREMENTS OF MOLDS 219 



quire only the pyrimidine and thiazole portions; and still others may 

 require only the pyrimidine or thiazole fractions, capable of syn- 

 thesizing whichever portion is lacking. Biotin, pyridoxine, p-amino- 

 benzoic acid, choline, and inositol are also required by some fungi. 

 Advantage has been taken of the essentiality of these substances for 

 the growth of certain fungi by various investigators to devise assay 

 methods because the growth is often proportional to the amount of 

 these substances in the culture medium. To a set of cultural vessels 

 containing a medium nutritionally complete in every respect, ex- 

 cept for the test substance for which the assay is being made, are 

 added varying quantities of a material with an unknown content 

 of the test substance. The degree of growth, being dependent on 

 the amount of the test substance, indicates the amount of the test 

 substance in the material. Similar assay methods have also been 

 devised for certain amino acids using molds as test organisms. The 

 principle of using molds for such assays has been employed to meas- 

 ure the available phosphorus and potassium in soil samples. 



Some very interesting and fundamental studies have been carried 

 out using mutants of Neurospora. Horowitz and Srb ^^ obtained 

 seven mutants of Neurospora crassa incapable of synthesizing 

 arginine, i.e., arginine had to be furnished these mutants for them 

 to grow. These mutants were obtained by exposing the parent or- 

 ganism to ultraviolet and x-radiations; presumably a certain gene or 

 set of genes was thus destroyed. Each of these mutants differed from 

 the normal by a different gene. One strain grew only when arginine 

 was added. Others grew on either arginine or citrulline, thus showing 

 that the latter could be converted to arginine. Others proved capable 

 of using arginine, citrulline, or ornithine. Thus it was deduced that 

 ornithine, too, could be converted to arginine. Each of the mutants 

 capable of utilizing ornothine was also able to use citrulline, whereas 

 the reverse was not always true. Because no strain was found 

 capable of utilizing ornithine and arginine but not citrulline, it was 

 concluded that citrulline was an essential intermediate in the syn- 

 thesis of arginine from ornithine. The importance of this type of 

 investigation with molds should not be underestimated. By such 

 studies, metabolic cycles as the above-described ornithine cycle of 

 Krebs and Hensleit occurring in higher biological forms can be read- 

 ily studied with the lower biological forms such as molds. The ap- 

 plications of such types of investigations are not limited to the fungi 

 themselves but are of use in studying the nutritional and metabolic 

 activities of higher biological forms. 



