120 CHEMISTRY OF PLANT LIFE 



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 pig- 

 ment 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. 



The chlorophylls are, of course, the regulator of photosyn- 

 thesis, absorbing solar energy with which the photosynthetic 

 process may be brought about. The simultaneous presence of 

 carotinoids in varying amounts undoubtedly serves to modify the 

 amount and character of the radiant energy absorbed, as these 

 pigments absorb a different part of the spectrum of light and hence 

 undoubtedly produce a different chemical activity or " actinic 

 effect " of the absorbed energy. The variations in depth of color 

 of foliage during different growing conditions, from a pale yellow 

 when conditions are unfavorable and growth is slow to the rich 

 dark green of more favorable conditions, is a famjliar phenomenon. 

 Whether this change in pigmentation is the result of an adjust- 

 ment of the plant protoplasm, so that it can absorb a more highly 

 actinic portion of the light, or is a direct effect of the lack of con- 

 ditions favorable to chlorophyll-production and active photosyn- 

 thesis, has not yet been determined. 



But there must be some influence other than response to en- 

 vironmental conditions which controls the vegetative color in 

 plants, since shrubs, or trees, which have green, yellow, red, and 

 purple leaves, respectively, will grow normally, side by side, under 



