2' 3| | |6 1' 2'
\ / \ / \ / \ /
\/ C \/ 5
4 | 4
O

The red dyes which were formerly obtained from madder, the powdered roots
of _Rubia tinctoria_, but are now almost wholly artificially synthetized,
consist of at least four different glucosides, the organic group of which,
in each case, is an hydroxy-derivative of anthraquinone. The most important
of these is _ruberythric acid_, composed of two molecules of glucose linked
with one of alizarin (1,2, dioxyanthraquinone). _Xanthopurpurin_ contains
1,3, dioxyanthraquinone, which is isomeric with alizarin; and _rubiadin_ is
a monomethyl (the CH_{3} being in the 4 position), derivative of this
compound. _Purpurin_ is a glucoside of 1,2,4, trioxyanthraquinone.

The soluble yellow pigments are generally glucosides of hydroxy-derivatives
of xanthone or flavone, known as oxyxanthones or oxyflavones. The sugars
which are united to these nuclei vary greatly, so that there are a great
variety of yellow, white, or colorless flavone or xanthone pigment
compounds. These compounds are almost universally present in plants. For
example, one typical set of examinations of the wood, bark, leaves, and
flowers of over 240 different species of tropical plants showed that
flavone derivatives were present in every sample which was tested, the
pigments being usually located in the powdery coating of the epidermis of
the tissues.

The following typical examples will serve to illustrate the composition and
properties of the glucosides of this type.

=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 rhamnos