THE B VITAMIN REQUIREMENTS OF ANIMALS AND PLANTS 307 



One might for comparative purposes wish to start a review with a 

 consideration of the B vitamin requirements of green plants. Most exist- 

 ing evidence indicates that green plants are able to meet their own vitamin 

 requirements by synthesis. Such plants, however, are seldom studied under 

 sterile conditions, and plant embryos and roots are known to require in 

 many cases a nutritional source of B vitamins ; hence this field of investi- 

 gation offers limitless opportunities for study. A brief consideration of 

 green plant requirements, therefore, occurs later in this chapter. 



Passing briefly to the bacteria whose B vitamin requirements have 

 been studied extensively, we find a broad spectrum of requirements. 

 While many bacteria are able to meet all their nutritional requirements 

 by synthesis, others require one or many of the known B vitamins, and 

 some are even more fastidious. 1, 2 The B vitamin requirements of bac- 

 teria have been extensively reviewed several times in recent years, 3 and 

 various aspects of bacterial and plant nutrition are considered in greater 

 detail elsewhere (p. 336) . As concerns the most "simple" living forms, 

 the viruses, we know so little of their metabolism and nutrition that 

 almost nothing can be said. 4 



Like the bacteria, the protozoa constitute a heterogeneous group nu- 

 tritionally as well as taxonomically. Many of the more plant-like forms 

 grow well in inorganic salts media, and have the ability to synthesize all 

 the B vitamins, while the so-called "higher" protozoa require most if 

 not all of them. Still others require as yet unidentified growth factors. 

 The nutrition of the protozoa has been intensively studied in recent 

 years, and excellent reviews are available. 5, 6 



Thus in passing from the higher plants to the higher animals, we pass, 

 in general, from cells requiring none to cells requiring most of the B 

 vitamins. At the juncture of the two kingdoms, we find single-celled 

 organisms that run the gamut of requirements, in general resembling nu- 

 tritionally the kingdom that they most resemble on the basis of other 

 conventional taxonomical considerations. If the line of demarcation be- 

 tween the plant and animal kingdoms is thus sharply drawn nutritionally 

 within the single-celled forms, we may expect that as we ascend the 

 evolutionary scale in the animal kingdom, no perceptible trend in B vita- 

 min requirements will occur, i.e., the lowest metazoan forms would re- 

 quire many if not most of the B vitamins. As shown in the discussion 

 which follows, the limited information available suggests that this is 

 essentially so. The recent development of methods for the study of the 

 biosynthetic abilities of developing embryos should even further assist 

 in examining this generalization in the years to come. 7 



The Lower Invertebrates. In considering the B vitamin requirements 

 of the lower invertebrates the absolute lack of evidence concerning the 



