54 General Botany 



leaves of different species of deciduous trees, from the cotton- 

 wood, 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. 



Red pigment in plants. 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 

 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 

 foHage 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 anthocyans are soluble in water, 

 as is shown by the red color of water in which beets have been 

 cooked. 



Autumn colors of leaves. In spring and summer the most 

 prominent feature of the landscape is the green color of the vege- 

 tation. The most striking feature in autumn is the varied colors 

 of the foliage on the trees and shrubs. In the northern provinces 

 of Canada most of the trees are evergreen, and the most abundant 

 deciduous trees, like the aspen, birch, and tamarack, merely turn 

 yellow. But in our Northern states the vivid greens of the sugar 

 maple, white oak, gum, and sumac disappear in a blaze of red 

 that contrasts strongly with the greens of the hemlock, spruce, 

 and pine. Every one who has seen the colors of autumn woods 

 and the annual falling of the leaves must have wondered what 

 processes go on within the leaves to bring about these changes. 



The development of the most brilKant red coloring of autumn 

 is commonly ascribed to the action of frost. This explanation is 

 probably incorrect, for careful observation indicates that the 

 color is most intense when a moderately low temperature is 

 accompanied by bright sunshine. In warm, cloudy autumns the 



