All of the natural anthocyanin pigments appear to contain a chlorine atom attached directly to the ring oxygen, as shown in the above partial formulas. In addition, they have four, five, or six hydroxyl (OH), or methoxy (OCH_{3}), groups attached at various points around the three rings. The following formula for _[oe]nidin_, one of the most complex of these anthocyanidins, will illustrate their structural arrangement. Cl | _____OCH_{3} O / \ HO__ / \ / \ _____/ \OH | | | \ / | | | \_____/ \ / \ / OCH_{3} OH OH C _Delphinidin_ is the corresponding compound without the two CH_{3} groups; while _cyanidin_ contains only five OH groups; and _pelargonidin_, only four OH groups. The anthocyanin pigments are soluble in water, alcohol, and ether, the solutions being red or blue in color according to the acidity or alkalinity of the medium. Their presence in many species of plants is hereditable, as these plants come true to color from seed, as in the case of red beets, red cabbage, several species of blue berries, etc. In other cases, the anthocyanin development depends largely upon the conditions of growth, particularly those which prevail during the later stages of development: 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,