ROBBINS : VITAMINS 123 



thiamine is deficient and cocarboxylase is not formed, carboxylase does not 

 function ; and the normal utilization of sugar does not occur. 



How specific are the vitamins? Vitamins are highly specific; that is, 

 nearly related compounds will not substitute for a particular vitamin. A small 

 change in the molecular structure of a vitamin reduces its efifectiveness, may 

 eliminate its activity entirely or even change it into a harmful compound. 

 These results are probably because of their function as coenzymes. 



What vitamins are important for plants? A dozen or more chemically 

 pure vitamins and similar substances are now available. Not all of these have 

 been demonstrated to be important for plants because usually a plant must be 

 discovered which is deficient for a vitamin before the need for it can be clearly 

 demonstrated. Nevertheless, deficiencies have been found for pantothenic acid 

 and para amino benzoic acid, the anti gray hair factors, for riboflavin, m-inosi- 

 tol, thiamine, biotin, pyridoxine and ascorbic acid. In the development of any 

 plant all these vitamins are probably essential, and othe'rs too, some of which 

 are still unidentified. Most plants, including all green plants and many of the 

 bacteria, yeasts and fungi, construct from sugar, minerals and a source of 

 nitrogen all the vitamins they require in amounts adequate for normal and 

 perhaps maximum development. Furnishing these plants with vitamins does 

 not improve their growth. 



Others suffer from one or more vitamin deficiencies : that is, they do not 

 develop satisfactorily in a medium which lacks vitamins. Some plants have a 

 complete deficiency for one or more vitamins. They are unable to synthesize 

 any of the vitamin (or vitamins) in question, and in its absence do not grow. 

 This is true of Phycomyces for thiamine. Others suffer from partial defi- 

 ciencies ; that is, they grow slowly in the absence of the particular vitamin, 

 but more rapidly if it is present in the medium. Apparently they are able to 

 make some of the vitamin, but not enough for maximum growth. Both com- 

 plete and partial deficiencies may be single (for one vitamin) or multiple (for 

 more than one vitamin). The deficiency may be absolute, or it may be condi- 

 tioned. By an absolute deficiency I mean that no known environmental con- 

 ditions enable the organism to synthesize the vitamin from the simple foods 

 and nutrients in a vitamin-free medium. This appears to be true of Phycomyces 

 in its relation to thiamine. Pythium butleri, on the other hand, suffers from a 

 thiamine deficiency in a concentrated mineral solution which is relieved by 

 diluting the solution. Its deficiency is conditioned by the medium in which it 

 is grown. 



The synthetic ability of a plant for a particular vitamin may be complete, 

 incomplete, or none; that is, some plants are able to construct the vitamin 

 from simple food and nutrients ; others are capable of making the vitamin if 

 supplied one or all of its intermediates ; and still others are incapable of con- 



