Plant Qrowth'Suhstances 



substance has not been tested, but it is easily hydrolyzed, and 

 probably has the same value as its equivalent of phenyl-acetic 

 acid. A few laboratory experiments have suggested that 

 glycine, when used as sole source of nitrogen in the growing of 

 plants, is fairly readily assimilable by plants, but such 

 experiments bear little relation to practice. Some may be 

 tempted to think that since phenaceturic acid comprises both 

 a growth-substance and a source of nitrogen, it might be an 

 especially valuable substance for plants, but it should be re- 

 membered that the amount of phenyl-acetic acid useful to the 

 plant is probably much less than a molecular equivalent of the 

 nitrogen required for growth. The growth-substance/nitro- 

 gen ratio in phenaceturic acid is i, when expressed in mole- 

 cules, but this is probably much too low for physiological 

 balance; if the plant is to take up from the phenaceturic 

 acid all the nitrogen it requires for growth, it will be left 

 with a possibly toxic excess of phenyl-acetic acid. The point 

 might be interesting to examine experimentally, but its 

 nutritional imphcations do not seem likely to be useful. 

 Phenaceturic acid, however, may find horticultural appHca- 

 tions as a source of phenyl-acetic acid in cases where growth- 

 regulation and not simple nutrition is the main object. 



Hydroxy-phenyl-acetic acid in the form of ^-hydroxy- 

 phenaceturic acid has been reported in urine after administra- 

 tion of hydroxy-phenyl-ethylamine, but its importance under 

 natural conditions is not known. It is almost certainly a 

 growth substance. (See p. 65.) 



Phenyl-propionic and p-^-hydroxy-phenyl-propionic acids 

 also have been reported in urine. 2 : 5-dihydroxy-phenyl- 

 acetic acid (homogentisic acid) occurs naturally in urine — in 

 the free state, apparently, not conjugated — in persons subject 

 to the rare condition known as "alcaptonuria". This is an 

 inborn error of metabolism, owing to which the sufferers are 

 unable to oxidize tyrosine and phenyl-alanine beyond the 

 stage of the 1:2:5 compound. Alcaptonurics do not appear to 

 suffer in health, but their urine blackens on exposure to air, 

 owing to the rapid oxidation of the dihydroxy-benzene 



radical. 



74 



