CHAPTER VII 



AROMATIC COMPOUNDS AND OXIDIZING ENZYMES 



The aromatic compounds may be defined as substances containing 

 the benzene carbon ring or a similar ring. A very great number occur 

 among the higher plants but of these many are restricted in distribution, 

 and may only be found in a few genera or even in one genus : others, 

 on the other hand, are widely distributed. At present our knowledge of 

 the part they play in general plant metabolism is obscure. 



The more widely distributed aromatic plant products may be grouped 

 as : 



1. The phenols, and their derivatives. 



2. The aromatic alcohols, aldehydes and acids (including the tan- 

 nins), and their derivatives. 



3. The flavone, flavonol and xanthone pigments, known as the soluble 

 yellow colouring matters. 



4. The anthocyan pigments, known as the soluble red, purple and 

 blue colouring matters. 



In connexion with the aromatic compounds it should be noted that 

 many of them contain hydroxyl groups, and one or more of these groups 

 may be replaced by the glucose residue, CeHnOa — , with elimination of 

 water and the formation of a glucoside, in the way already described 

 (see p. 48). The majority of such compounds are sometimes classed 

 together as a group — the glucosides — regardless of the special nature of 

 the substance to which the glucose is attached (this course has been 

 followed to some extent in Chapter IX with compounds, the chief 

 interest of which lies in their glucosidal nature). In treating of the 

 aromatic substances in the following pages, mention will be made when 

 they occur as glucosides, this combination being in these cases only a 

 subsidiary point in their structure. 



The various groups of aromatic substances will now be considered 

 in detail. 



Phenols. 



There are three dihydric phenols, resorcinol, catechol and hydro- 

 quinone, but of these only the last is known to exist in the free state in 

 plants. The two former frequently occur as constituents of complex 

 plant products, and may be obtained on decomposition of such complexes 

 by fusion with strong alkali, etc. 



