Leaf Coloration and the Fall of Leaves 65 



that the green pigment disappears from the leaves of most 

 deciduous plants and unmasks the yellow pigments in the 

 chloroplasts. There is every gradation in the readiness with 

 which the green pigment disappears from the leaves of differ- 

 ent species of deciduous trees, from the Cottonwood, in which 

 the leaves become yellow during a midsummer drought, to 

 the peach, in which they may still be vivid green when shed. 

 In evergreens the chlorophyll is less sensitive and external 

 conditions are not so effective in causing changes in the color 

 of the leaves. 



The red pigment. The red colors of autumn leaves are not 

 due to changes in the content of the chloroplasts, but to the 

 formation in the cell sap of a red pigment called anthocyan. 

 This same pigment is present in the cells of many young leaves 

 in early spring. It occurs also in beets, in red cabbage, in the 

 petioles and veins of many different kinds of leaves, in the 

 Coleus and other foliage plants, and in many flowers. The 

 presence of anthocyan in the cell sap makes the whole cell 

 red, and any or all of the cells may develop the pigment. 



The development of the most brilliant red coloring of au- 

 tumn is commonly ascribed to the action of frost. This ex- 

 planation is probably incorrect, for careful observation indi- 

 cates that the color is most intense when a moderately low 

 temperature is accompanied by bright sunshine. In warm, 

 cloudy autumns the colors are more likely to be dull, with the 

 yellows predominant. That sunlight is important in the 

 development of the red pigment may be shown also by an 

 examination of a leaf that has been closely shaded by another. 

 The pigment stops so abruptly where the shade begins that 

 a perfect print of the uppemiost leaf results. 



The red colors of the fruits of peaches, apples, and pears 



