UREA IN PLANTS 279 



Uric acid, which is systematically named 2:6:8 trioxy- 

 purine, has the formula 



NH CO 

 CO C NH 



. I I > 



NH C NH 



It does not occur in plants, but is a well-known product 

 of metabolism in the animal world. In view of the close 

 relationship between this substance and the other purine bases, 

 the assumption does not seem unwarranted that the purine 

 bases in the plant are also waste products (see below). And, 

 in this connexion, it is interesting to find that the presence of 

 urea, in very small amounts, has been observed by Fosse * in the 

 higher plants. To what extent this substance is a physiological 

 product of the cell is doubtful. 



The identification of individual members of the purine bases 

 is not very easy, although the recognition of a purine base as 

 such is rendered simple by the so-called murexide test which 

 is given by practically all the members of this group of com- 

 pounds. 



The test consists in evaporating the substance (uric acid 

 or caffeine may be used) in a porcelain basin with dilute nitric 

 acid over a water bath. A yellowish residue remains which 

 on the addition of ammonia or by exposure to ammonia vapour 

 turns pink ; potash changes the colour to purple. 



The identification of caffeine in plants has been the subject 

 of numerous researches f ; it is precipitated by several alkaloidal 

 reagents from solutions containing concentrated hydrochloric 

 ecid, but not from neutral solutions ; these precipitates are, 

 however, not characteristic. Behrens J has described methods 

 of identifying this substance with the help of mercuric chloride 

 and of silver nitrate and nitric acid. The method is as 

 follows : 



* Fosse: " Compt. rend.," 1912, 155, 851; 1913, 156, 567. 93 8 J 157, 94 8 5 

 1914, 158, 1374; 159, 253; "Ann. Chim.," 1916, [ix], 6, 13, 155- See Ver- 

 schaffelt: " Pharm. Weekblad," 1914, 51, 189, lor summary of work on urea in 

 plants. 



fClautriau: "Nature et Signification des Alcaloi'des vgetaux," Brussels, 

 1900. 



J Behrens : " Anleitungen z. 'mikrochemisc'nen Analyse d. wichtigsten organ. 

 Verbindungen," 1897, IV, 14. 



