e, 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.

The following red or blue plant pigments, which are anthocyan
glucosides, have been isolated and studied (for the structural
arrangement of the characteristic groups, see pages 116): from
cornflower and roses, _cyanin_, C_{28}H_{31}O_{16}Cl (2 molecules
glucose + cyanidin); from cranberries, _idain_, C_{21}H_{21}O_{10}Cl
(galactose + cyanidin); from geranium, _pelargonin_, C_{27}H_{30}O_{15}Cl
(2 molecules glucose + pelargonidin); from paeony, _paeonin_,
C_{28}H_{33}O_{16}Cl (2 molecule