=Quercitrin=, C_{21}H_{20}O_{11}, 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). =Quercetin=, C_{15}H_{10}O_{7}, 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=, C_{21}H_{20}O_{12}, is derived from the same flavone, but contains glucose instead of rhamnose, as the sugar constituent of the glucoside. =Apiin=, C_{26}H_{20}O_{9}, 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 \ COH·CHOH·CHO), and apigenin, which is a 1,3,4',trioxyflavone. / CH_{2}OH =Xanthorhamnin=, C_{34}H_{42}O_{20}, 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 CH_{3} 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 isomeric with similar flavone and xanthone derivatives, and the transition from one color to the other in plants takes place very easily under the action of oxidizing or reducing enzymes. This accounts for the change of reds and blues to yellows and browns, and vice versa, under changing temperature conditions.