THE NATURE OF VITAMIN REQUIREMENTS 327 



which the vitamin is essential will obviously reduce the demand for 

 the exogenous vitamin. The replacement of nicotinic acid by large 

 amounts of tryptophan represents replacement by a precursor (20). 

 The replacement of p-aminobenzoic acid by methionine (269) may be 

 interpreted as evidence that at least one function of the vitamin is in 

 the biosynthesis of methionine, and a similar mechanism is invoked to 

 explain the partial replacement of biotin by aspartic acid (190). 



The third type of conditioned requirement is the most interesting 

 and the least studied: the possibility that fungi change in their syn- 

 thetic capacity and in their vitamin needs during development. It 

 must be realized that the typical vitamin experiment performed with 

 fungi actually measures in the first instance the capacity of the spores 

 to germinate in the absence of the vitamin in question; if the spore 

 germination process requires the growth factor, the organism as such is 

 recorded as having the requirement. It is entirely possible that spore 

 germination may require factors which mature mycelium can synthe- 

 size for itself. Thus, the growth of Memnoniella echinata from a spore 

 inoculum requires biotin, which is not replaceable by desthiobiotin. 

 However, if desthiobiotin is added to a growing culture supplied with 

 limiting amounts of biotin, the desthiobiotin elicits a significant 

 growth response (191). Evidently, the mycelium is able to convert 

 desthiobiotin to biotin but the spore cannot. Successful use of the dif- 

 ferential germination method for the detection of vitamin-deficient 

 mutants (69, 308) shows that spore germination is affected by growth 

 factor deficiencies. 



Several fungi — Alternaria solani (138), Fusarium solani (285), and 

 Penicillium digitatum (61) — are reported to be accelerated in early 

 growth by vitamins the requirements for which are not evident if total 

 growth over a long period is measured. In Myrothecium verrucaria a 

 requirement for biotin is detectable only at a stage shortly after spore 

 germination (157). 



Among the dimorphic fungi (Chapter 1), a single study (229) sug- 

 gests that the yeast and the mycelial phases of some forms differ in their 

 biotin requirement. 



The onset of reproduction as a phase of growth may, although the 

 data are far from decisive, involve changes in vitamin needs and re- 

 sponses. There is, however, no evidence of a singular association of 

 one vitamin with sporulation per se, independent of growth. This 

 problem is considered in Chapter 1 1 . 



Multiple requirements are especially common in the yeasts; five or 

 six vitamins may be needed (27, 29, 39). Among the filamentous fungi, 

 there are several instances of a requirement for three different vitamins, 



