84 CHEMISTRY OF PLANT LIFE 



Quercitrin, C2iH2oOn, is found in oak bark, in the leaves of 

 horse-chestnut, and in many other plants, often associated with 

 other pigments. It is a brilliant yellow crystalline powder. 

 Industrially, it ranks next to indigo and alizarin in importance as a 

 natural dye stuff. It is a glucoside of rhamnose with 1, 3, 3', 4', 

 tetraoxyflavonol (i.e., the flavone nucleus with five OH groups 

 replacing the hydrogens in the 1, 3, 5, 3', and 4' positions). Quer- 

 cetin, CisHioOr, which is the tetraoxyflavonol itself, without any 

 sugar in combination with it, is found in the leaves of several 

 species of tropical plants and in the bark of others. Isoquercitrin, 

 C2iC2oOi2, is derived from the same flavone, but contains glucose 

 instead of rhamnose, as the sugar constituent of the glucoside. 



Apiin, C26H2oOg, the yellow glucoside found in the 

 leaves of parsley, celery, etc., contains apiose (a pentose 

 sugar of very unusual structure, represented by the formula, 

 CH 2 OH V 



yOH-CHOH-CHO), and apigenin, which is a 1, 3, 4', 



CH 2 oir 



trioxyflavone. 



Xanthorhamnin, Cs4H4202o, is a very complex glucoside 

 containing two rhamnose and one galactose groups, united with 

 rhamnetin, which is quercitin with the H of the OH in either the 

 1, or 3, position replaced by a methyl group. There are several 

 similar pigments which differ from xanthorhamnin only in the 

 number or position of the methoxy groups (i.e., the OH groups 

 with a CHs replacing the H), or in the nature of the sugar which is 

 present in the compound. Rhamnetin itself is found in the fruits 

 of certain species of Rhamnus, and is used in dyeing cotton. 



The structural arrangement of the characteristic groups of 

 these flavone pigments will be dealt with more in detail in the 

 chapter dealing with Pigments (Chapter VIII). 



The best-known yellow pigment which is a xanthone derivative 

 is euxanthic acid, known as " Indian yellow," which is a "paired" 

 compound of glucuronic acid (see page 42) and euxanthone. The 

 latter is a 2, 3', dioxyxanthone. The pigment is found in the urine 

 of cattle which have been fed on mango leaves. 



The soluble red, blue, and violet pigments are glucosides of 

 various hydroxy-derivatives of the anthocyan nucleus. Their 

 constitution and properties will be discussed in detail in the 

 chapter dealing with the Pigments. These compounds are iso- 



