HYBRID NUTRITIONAL REQUIREMENTS 115 



Does Phycomyces fail to grow in the basal solution because of the absence 

 of something essential which it needs for growth, or because of the presence 

 of something detrimental? Does Aspergillus niger grow in the basal solution 

 because it does not need to be furnished with the "essential" substance, or 

 because it is more resistant to the supposed injurious ingredient? 



For the example cited, we have a definite and well demonstrated explana- 

 tion. Phycomyces fails to grow in the basal medium because it requires the 

 vitamin, thiamine — which it is unable to make from sugar, mineral salts, and 

 asparagine. Aspergillus niger also needs thiamine, but it constructs the vita- 

 min from the elementary materials present in the basal solution. In this in- 

 stance, therefore, the failure to grow is due to the lack of something es- 

 sential for growth; namely, thiamine, the precursor of co-carboxylase. 



This is not an isolated example. Many species of fungi grow slowly, or not 

 at all, in a basal medium because of their inability to make one or more of the 

 essential metabolites. These metabolites may include various vitamins, 

 purine and pyrimidine bases, amino acids, fatty acids, or substances as yet 

 unidentified. 



ESSENTIAL METABOLITES-RELATION TO GROWTH 



It may be assumed that the complex chemical compounds which make 

 up the cell substance of a living organism are constructed by the organism 

 from simpler compounds. A series of intermediate chemical compounds are 

 formed between the original simple foods and nutrients and the final product, 

 cell substance. This step-wise progression from simple to complex is made 

 possible by a series of enzymes, also made by the organism, which operate on 

 each stage as that stage is completed. Although synthesis is likely to be 

 emphasized in considering growth, there are other subsidiary processes — 

 necessary concomitants for the building up of new cell substance. The cata- 

 bolic processes of digestion and respiration also occur in steps, and are made 

 possible by the action of a series of enzyme systems. 



Any substance playing a necessary part directly or indirectly in the chain 

 of reactions which end in the synthesis of new cell substance is an essential 

 metabolite. Unless each essential metabolite, each chemical substance in the 

 step-wise process of growth, each enzyme which facilitates the chemical re- 

 actions concerned, is made within the organism or supplied from without, the 

 series is interrupted. New cell substance is not made, and growth does not 

 occur. If not enough of an essential metabolite is made, growth will be 

 slowed. 



Of course, this is an oversimplified statement of a very complicated 

 process. The reactions concerned in growth probably do not occur in a 

 straight line. Some steps may be bypassed and side reactions may occur, all 

 of which may affect the speed and character of the growth which results. 



It would be difficult to estimate the number of essential metabolites in- 



