472 PLANT GROWTH SUBSTANCES 



We can summarize briefly by saying that, with the exception of 

 embryos, investigations generally indicate that higher plants have no 

 complete or partial requirements for amino acids. 



This does not mean, however, that higher plants are unable to absorb 

 amino acids as such and utihze them. Numerous investigations on the 

 direct utilization of organic nitrogen by higher plants beginning with 

 those of Lutz (30) in 1898 have demonstrated that some amino acids 

 supplied under sterile conditions can be absorbed and utiHzed under some 

 circumstances. There is disagreement, however, as to which amino acids 

 can be assimilated by intact plants and the conditions under which 

 they are utilized are ill-defined. Hutchinson and Miller (21) in summariz- 

 ing the literature in 191 1 said that more or less satisfactory evidence of 

 assimilation had been obtained for leucine, aspartic acid, asparagine, and 

 tyrosine. The gains of nitrogen were, however, generally very small, 

 and in many cases negative results were obtained. With some modifica- 

 tion in the list of specific amino acids, the situation nearly 40 years later 

 is about the same. 



For almost any amino acid for which utilization is claimed, other 

 evidence showing that it is not utilized or is toxic can be cited. Aspartic 

 acid, for example, is reported by Molliard (32) to be assimilated by 

 radish; Beaumont et al. (i) obtained negative results with tobacco; 

 Hutchinson and Miller (21) found it to be a fair source of nitrogen for 

 peas; Tanaka (54) states that it is not utiHzed by Sisyrinchium; Virtanen 

 and Linkola (57) found it to be assimilated by peas and clover but not by 

 wheat and barley for which it was injurious. Brigham (11) states that 

 asparagine is superior to nitrates for dent corn; Beaumont et al. (i) 

 found asparagine to be a fair source of nitrogen for tobacco; Steinberg 

 (53) reports it to be quite toxic for tobacco. White (59) in 1937 concluded 

 that 9 amino acids were essential for the growth of excised tomato roots 

 and called attention to the close correspondence between the amino 

 acids essential for the growth of rats, for diphtheria bacilli, and for 

 tomato roots. Two years later (61) he reported that the 9 essential 

 amino acids could be replaced by glycine which was not included in 

 the original group. Bonner (7) and Day (13) were unable to demonstrate 

 beneficial effects of glycine. We have grown excised tomato roots through 

 137 passages extending over almost 13 years in a medium in which the 

 only organic constituents were sugar and thiamin and the only source 

 of nitrogen was nitrate. 



