s glucose + paeonidin, a monomethyl
cyanidin); from blue grapes, _[oe]nin_, C_{23}H_{25}O_{12}Cl (glucose +
[oe]nidin); from whortle berry, _myrtillin_, C_{22}H_{23}O_{12}Cl
(glucose + myrtillidin); from larkspur, _delphinin_, C_{41}H_{39}O_{21}Cl
(2 molecules glucose + 2 molecules _p_-oxybenzoic acid + delphinidin);
and from mallow, _malvin_, C_{29}H_{35}O_{17}Cl (2 molecules glucose +
malvidin).

The blue dye, indigo, is derived from a glucoside of an entirely different
type, known as _indican_. Indican is readily extracted from the leaves of
various species of indigo plants. When hydrolyzed, it yields glucose and
_indoxyl_ (colorless). Indoxyl is easily oxidized to _indigotin_ (the deep
blue dye known as "indigo"). The equations illustrating these changes are
as follows:

(_a_) C_{14}H_{17}O_{6}N + H_{2}O = C_{6}H_{12}O_{6} + C_{8}H_{7}ON
Indican Glucose Indoxyl

(_b_) 2C_{8}H_{7}ON + O_{2} = C_{16}H_{10}O_{2}N + 2H_{2}O
Indoxyl Indigotin

The structural relationships of indoxyl and indigotin may be illustrated by
the following formulas:

O O
/\ /\ | | / \
/ \___COH / \___C C____/ \
| | | | | | | | |
| | C-H | | C=C | |
\ /\ / \ /\ / \ / \ /
\/ N \/ N N \/
| | |
H H H

Indoxyl Indigotin

Natural indigo dye is prepared by fermentation of indigo leaves, the decay
of the cell-walls liberating the enzymes in the tissues, which bring about
the chemical changes illustrated in the above equations.

THE CYANOPHORE GLUCOSIDES

Several glucosides which yield hydrocyanic acid as one of the products of
their hydrolysis are of common occurrence in plants. These are generally
spoken of as the "cyanogenetic" glucosides; but as they do not actually
produce cyanogen compounds, but only liberate them when hydrolyzed, the
recently suggested term "cyanophore" undoubtedly more correctly indicates
their properties.

The best known and most widely distributed of these