80 THE BIOCHEMISTRY OF B VITAMINS 



knowledge it is impossible to interpret with any certainty the action on 

 plant growth of animal manures and decaying organic matter — materials 

 which always contain substantial amounts of B vitamins. 



Many experiments have been conducted to ascertain whether or not the 

 B vitamin content of a crop can be increased by using natural fertilizers. 

 In general, the effect, if any, has been small. 15, 16, 17 In some experiments, 

 at least, larger crop yields (which means larger total vitamin yields) have 

 been obtained from plots treated with manure. Of course, it is unsafe to 

 ascribe the effect of a natural manure to its vitamin content without 

 further investigation. More often than not, investigations in this general 

 field have been directed toward practical ends and have not been designed 

 to answer fundamental biochemical questions. 



It should be pointed out in this connection that, in higher organisms 

 generally, amounts of thiamine beyond an organism's needs are not pro- 

 duced or stored in quantity, nor are they physiologically valuable or 

 active. Without an ample supply, however, an organism cannot live. 

 Hence, the finding that plants grown under different cultural conditions 

 always contain about the same amount of thiamine throws more light on 

 the question of how thiamine functions than upon how it originates. 



There are several well authenticated facts which have a bearing on the 

 problem of how thiamine is produced during the growth of green plants 

 and the extent to which microorganisms participate. (1) Some plant roots 

 (tomatoes) are tremendously stimulated in growth by very low concen- 

 trations of thiamine (0.0001 /xg per ml or less) 18 such as may occur in the 

 soil and originate in the bacteria and molds present. (2) Mycorhizal fungi, 

 which may be either "endotrophic" or "exotrophic," not only play an 

 indispensable role in the life of such plants as orchids, but are likewise 

 always associated with many common flowering plants and with many 

 forest trees (conifers, oaks, beeches, alders, willows, poplars, etc.) . In 

 many of these latter the mycorhiza are essential to continued life, and 

 the production of specific B vitamins such as thiamine by the mycorhizal 

 fungi is not ruled out as an important factor in the symbiotic relationship. 

 (3) During the early stages of growth of a seedling, especially if the seed 

 is small and therefore contains little storage food, the seedling is likely 

 to respond to tissue extracts and vitamin supplements by increased 

 growth. (4) Pea seedlings grown 8 days in the dark contain one-half to 

 one-third as much thiamine as those grown in the light, as determined by 

 the Phyeomyces test. 19 (5) Certain plant roots (tomato) are capable of 

 synthesizing the pyrimidine portion of thiamine and supplying their 

 thiamine needs if the thiazole portion alone is supplied. 20 (6) The bacteria 

 found in the "rhizosphere" of plants are reported to have more growth 

 factor requirements than those more distant from the plant roots. 21 



