Plant Qrowth'Suhstances 



Handbuch). The acid gives a deep red colour with Millon's re- 

 agent. 



Tyrosine (/)-hydroxy-phenyl-alanine). — Not a plant- 

 growth substance while intact. For tests, see textbooks. 



For properties and methods of isolation of some aromatic 

 hydroxy-acids, see the sections by Waser (1923), and by 

 P. Brigl, in Abderhalden's Handbuch, 1931, Abt. 4, Teil 5(1), 

 483-512, and Baumann's (1882) method on pp. 510-512 of 

 the latter. 



Homogentisic acid. (2 : 5-dihydroxy-phenyl-acetic acid.) — 

 The claim of this body to be considered a growth-substance is 

 not certain, but it may be useful to give a few hints for its 

 detection and estimation, as a guide to adoption of methods to 

 search for it in agricultural materials. Readers who are especi- 

 ally interested cannot do better than consult Weise's mono- 

 graph (see p. 75). 



From urine containing homogentisic acid and shaken with 

 sodium hydroxide solution and benzoyl chloride, di-benzoyl-di- 

 hydroxy phenyl-acetamide (C gH 5. CO O )2. C gHg. CHg. CONHg 

 separates out, which can be recrystallized from hot ethyl 

 alcohol. M.P. 203-204°. This derivative is characteristic of 

 alcaptonuric urine, but needs to be differentiated from 

 tribenzamide, M.P. 201-205°, which is Hable to form in small 

 amounts when urine is benzoylated. Tribenzamide, however, 

 is difficultly soluble in hot alcohol. 



The lead salt of homogentisic acid is useful for isolation; 

 it crystallizes in colourless or brownish prisms of M.P. 214°. 

 It is insoluble in alcohol and ether, and only very slightly 

 soluble in water. 



With ferric chloride solution, homogentisic acid, even in 

 fairly dilute solution, gives a transient blue colour. With 

 Millon's reagent, an aqueous solution of the acid gives a 

 yellow colour; on standing, a yellow precipitate develops in 

 the cold, which turns red on heating. 



A peculiar titration method has been evolved by Baumann 

 (1891). This depends on the reduction of N/io silver nitrate 

 solution in urine made alkaline with ammonia. To 10 ml. of 

 urine are added 10 ml. of 3 per cent, ammonia solution, and 



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