WILLIAM J. ROBBINS 471 



special attention must be given to cleanliness of glassware and other 

 utensils and to purity of chemicals. Effective quantities of one or more 

 vitamins may be present in the carbohydrates (maltose of adequate 

 purity is especially difficult to obtain), in agar, in gelatin, in cotton, cheese 

 cloth, asparagine, or any other product of natural origin. We have found 

 some samples of the vitamin thiazole contaminated with the vitamin 

 pyrimidine. 



Amino Acids 



As far as is known higher plants have no amino acid requirements in 

 the sense that they are heterotrophic for a particular amino acid or group 

 of amino acids and will not grow unless suppHed from without with one 

 or more of these metabolites. Higher plants are able to synthesize all 

 the amino acids included in their cell substance from inorganic nitrogen, 

 for example, nitrates or compounds of ammonia. In tact, it is probable 

 that any cell of at least some species of higher plants can construct amino 

 acids from inorganic compounds of nitrogen. This is suggested by the 

 demonstration that some kinds of excised roots (42,7) and stem tissues 

 (12,27,39) grow in media in which nitrates are the only source of 

 nitrogen. 



With the possible exception of embryos (51) and perhaps some stem 

 tissues (16), higher plants appear to synthesize amino acids from inorganic 

 nitrogen as rapidly as they can be used in the plant's metabolism. The 

 burden of evidence indicates that better growth is obtained with in- 

 organic nitrogen than with any amino acid or mixture of amino acids. 

 Furthermore, most investigations fail to demonstrate beneficial effects 

 from the addition of one or more amino acids to a solution containing 

 inorganic nitrogen if the plant is adequately supplied with carbohydrate 

 through photosynthesis or otherwise. 



I say, as far as is known, higher plants have no amino acid requirements. 

 Not all of them, of course, have been investigated, and it is possible that 

 such parasites as dodder or saprophytes like Indian pipe, have amino 

 acid deficiencies. This question cannot be answered until these plants 

 are successfully grown on a medium of known composition. Neither can 

 we assert that all roots grow as tomato roots do with nitrates as the only 

 source of nitrogen. It is conceivable that some of the roots which have 

 not been successfully cultivated in excised condition may have amino acid 

 requirements. The same may be said for stem tissues. 



