As a rule, the most brilliant of these yellow pigments are found in the largest quantities in the bark and wood of various species of tropical plants; although they are also present, in smaller amounts, in the blossoms of species growing in temperate zones. The anthoxanthins are easily converted into anthocyanins, and _vice versa_, by the action of oxidizing and reducing enzymes which are commonly present in the tissues of the plants which develop the pigments. THE PRODUCTION OF ORNAMENTAL PIGMENTS IN FLOWERS, ETC. The breeding of flowering plants having blossoms of almost any desired color has become a commercial enterprise of large importance. The results which have been obtained, in many cases, have been made the object of scientific study of the genetics of color inheritance. These studies have developed certain interesting facts with reference to the chemistry of the development of these ornamental pigments, which may be briefly mentioned here. In many of the plants which have been studied, the color of the flowers depends upon several different factors, as follows: _C_, a chromogen (or color-producing substance) which is generally a flavone or xanthone glucoside, and which may be either yellow or colorless. _E_, an enzyme which acts upon _C_, to produce a red pigment. _e_, another enzyme which acts upon the red pigment, changing it to some other anthocyanin color. _A_, an antioxidase, or antienzyme, which prevents the action of _E_. _R_, an enzyme which changes reds to yellows. Thus, if a plant whose flower contains only the factor _C_ be crossed with one which contains the factor _E_, a red blossom will result, or if it contains the factor _e_ more intense pigments are developed. But if either _A_ or _R_ are present, no change in the color of the original parents will result from the crossing. THE PHYSIOLOGICAL USES OF PIGMENTS The vegetative pigments undoubtedly serve as agencies for regulating the rate of metabolic processes. At the same time, it is extremely difficult to determine whether the presence of a pigment in any given case is the cause or the effect of the changes in the plant's activities which result from changes in its external environment.