Plant Qrowth'Suhstances 



posed meat, on account of the presence of indole and other 

 substances in the mixture, are too complicated to be given 

 here, but they should be borne in mind. 



Indole-propionic acid. — Ferric chloride added to a dilute 

 solution of the acid gives a white turbidity, which becomes red 

 on heating. Concentrated potassium nitrite and acetic acid 

 added to a saturated solution in water form fine yellow needles 

 of a nitroso compound, which melts at 135° with evolution of 

 gas. 



Tryptophan. — Not a growth-substance^ when intact. For 

 usual tests see textbooks. 



An important modification of the well-known Adamkiewicz- 

 Hopkins reaction has been made by Winkler (1934). This 

 reaction depends on the blue-violet colour developed when 

 glyoxylic acid (often present as an impurity in acetic acid) and 

 concentrated sulphuric acid are added to free or combined 

 tryptophan solutions. Winkler has shown that the test is 

 inadequately given, or even fails, unless a trace of copper 

 salt (conveniently, sulphate or acetate) is added before the 

 sulphuric acid. The glyoxylic acid should be roughly equi- 

 molecular with the tryptophan. Under conditions described, 

 the modified reaction is quantitative for free tryptophan at 

 least. 



For a method of detection of free and combined tryptophan 

 in plants by nitration, see Roth (1939). 



Creatine and creatinine. — For properties and tests, see text- 

 books on human biochemistry. Creatinine was recognized by 

 Schreiner et al. (191 1, E) by means of its zinc chloride com- 

 pound. 



Oxal-acetic acid. — This is not known to be a growth- 

 substance in the current sense, but it is assumed by Virtanen 

 and Laine (1937) to be an intermediate in the biological 

 fixation of nitrogen, particularly in leguminous plants. In 

 either aqueous or alcoholic solution, both forms of the acid 

 give an intense red colour with ferric chloride solution, where- 

 fore it may perhaps be confused with indole-acetic acid. 

 Oxal-acetic acid, however, has a peculiar reaction : in presence 



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