474 PLANT GROWTH SUBSTANCES 



effective because it furnishes a carbon configuration rather than because of its 

 nitrogen content is another factor to be considered. 



Miscellaneous Factors. It is hardly necessary to mention that temperature, 

 hght intensity, the pW of the medium, and other important environmental 

 factors may affect the action of amino acids. Riker and Gutsche (39) have 

 emphasized the significance of the concentration of an amino acid in determin- 

 ing its effects. 



Importance of Research on Amino Acids. — Interest in the relation of 

 higher plants to amino acids has extended in various directions. There 

 has been a substantial amount of research on the possibility that organic 

 fertilizers owe their effects in part to the absorption of nitrogen in 

 organic form including amino acids. The burden of evidence indicates 

 that except in particular circumstances the direct absorption of amino 

 acids from organic fertilizers is of little or no importance. The inhibitory 

 action of particular amino acids and the balance between them has 

 also received attention. It is suggested that an excess of free amino 

 acids in protein metabolism may be responsible for the symptoms of 

 specific plant diseases, that is, isoleucine and frenching in tobacco (53); 

 that amino acids may act as growth-regulating substances (39) or as 

 cofactors for auxin (8). The amino acid balance is considered of im- 

 portance in determining the growth pattern of a plant (57) and in the 

 problem of differentiation, dormancy, and interspecific crosses (51). 



It seems odd that there is so little evidence that amino acids are as 

 good or better for higher plants than inorganic nitrogen. If amino acids 

 are synthesized from inorganic nitrogen and then condensed stepwise 

 through proteoses, peptones, and polypeptides to protein, one would 

 expect that suitable mixtures of amino acids should be as effective or more 

 effective than inorganic nitrogen. Is the evidence incomplete? Are 

 amino acids absorbed and translocated too slowly, or do proteins arise 

 by the condensation of units of different and perhaps simpler form than 

 amino acids? 



Although substantial progress has been made in our knowledge of the 

 vitamin and amino acid requirements of higher plants, it is evident that 

 what we now know is a beginning only. Further work on intact plants, 

 embryos, and excised tissues aimed at defining growth-substance re- 

 quirements and including all types of essential metabolites will doubtless 

 elucidate many of the questions which are still unanswered. 



