246 P!a7it Biology 



crystals. It is widely distributed in the green parts of plants, but it is 

 also found in flowers, fruits, seeds, roots, and certain fungi. It is present 

 in large quantities in carrots. The carotene content of leaves varies 

 with the seasons because its formation is dependent on light. Its func- 

 tion is not clear, but its tendency to unite with oxygen may be significant 

 in photosynthesis where reduction of compounds containing oxygen 

 occurs. 



Flavones (L. flavus, yellow) are yellow pigments in such plants as 

 yellow wood (Morus), osage orange (Madura,) and sumac (Rhus). 

 They are not so common in yellow flowers and leaves where the color is 

 due to carotinoids. Flavones are probably oxidation products, the exact 

 functions of which are not clear at present. They are responsible for 

 the yellow color of onion skins and certain snapdragons. In most plants 

 they occur as glucosides. 



Red pigments known as anthocyanins (Gr. anthos, flower; kyanos, 

 dark blue) are dissolved in the cell sap of such structures as certain 

 flowers, fruits, and leaves, beet roots, red cabbage, etc., where they give 

 red, purple, or bluish colors. Anthocyanins absorb some light energy 

 which is converted into heat. The latter increases the temperature, 

 which accelerates the metabolic activities of the cell and probably aids 

 in protecting the plant from the lowered temperature of the surrounding 

 air. This is plausible in view of the fact that anthocyanins are more 

 common in leaves in the fall than in summer. Anthocyanins develop 

 more abundantly in all parts of a high alpine plant than in the lowland 

 plant, even in plants of the same species. For example, the common 

 weed, the yarrow (Achillea millefolium), has white flowers in lowlands 

 and southern regions but has red flowers on high mountains and in the 

 far north. Anthocyanin formation depends on ( 1 ) the presence of 

 sugars, (2) a certain amount of light (they are "sun pigments"), al- 

 though they also develop in the roots of beets and in the outer part of 

 radish roots which do not contact light; and (3) a lower temperature, 

 which naturally must be above freezing. A few examples of anthocya- 

 nins which occur naturally are red plums, red bananas, red rose, red ber- 

 ries, red geranium, red hollyhock, red hyacinths and tulips, the bracts 

 of the Poinsettia, the scarlet oak, scarlet maple, and similar materials. 



In general, plant pigments have been credited with such functions as 

 follows : ( 1 ) An aid in respiration. The relationship between anthocya- 

 nins and easily oxidizable sugars suggests a possible correlation between 

 the processes of oxidation and respiration. Carotene is oxidizable into 

 xanthophyll. Red anthocyanins may be changed to blue ones by oxidiz- 



