lopment: as
in the case of apples, where the amount of red color in the skin depends to
a large extent upon the conditions under which the fruit ripens.

Anthocyanin pigments often make their appearance late in the season; in
fruits, etc., as the result of the normal ripening process but in leaves as
the result of shorter daylight illumination accentuated also by sharp
frosts.

THE ANTHOXANTHINS

The yellow plant pigments, other than the carotinoids, are almost without
exception glucosides having a xanthone or flavone nucleus. These typical
nuclei are illustrated on page 83. In these nuclei, as in the anthocyan
one, the oxygen atom is strongly basic and combines with mineral acids to
form salts (the oxygen becoming quadrivalent) and the color of the pigment
depending upon the nature of the combination formed in this way.

The anthoxanthin pigments are yellow, crystalline solids, which are only
slightly soluble in water. They dissolve readily in dilute acids and
alkalies, giving yellow or red solutions which are of the same color in
either acid or alkaline media. They are extensively used as yellow dyes.

Many of the common members of this group have been mentioned in the chapter
dealing with the glucosides. The characteristic pigment nucleus of several
of these is as follows:

_Chrysin_, found in various species of poplar and mallows,

O _____
HO / \ / \ __/ \
| | | \_____/
| | |
\ / \ /
HO C
|
O

_Apigenin_, found in parsley and celery, as the glucoside apiin,

O _____
HO / \ / \ __/ \OH
| | | \_____/
| | |
\ / \ /
HO C
|
O

_Campferol_, found in Java indigo, as the glucoside campferitrin,

O _____
HO / \ / \ __/ \OH
| | | \_____/
| | |OH
\ / \ /
HO