32 LEAF COLORATION 



a variety of other colors often appears. One of the most common 

 is red. Young shoots of the rose, grape vine and so much of the 

 early spring vegetation are tinged with red. So in the fall our 

 vegetation takes on a greater wealth of coloration than is seen 

 in any other land. And finally there are leaves variegated with 

 yellow, white and red, colored fruits and other organs, and the 

 endless hues of the flowers. These colors are caused by pigments 

 or by chromoplasts (p. 2) that are developed in certain cells and 

 which either transmit or reflect to the eye their particular colors. 

 The white blotches of variegated leaves are caused by the absence 

 of chlorophyll, thus allowing the light to pass practically un- 

 changed, or in some cases these areas are characterized by large 

 intercellular spaces filled with air. Such a structure would tend 

 to reflect the light and so contribute to the white appearance of 

 the area (p. 39). In some cases these colors may be of service to 

 the plant. The red probably functions as a screen to young or- 

 gans, shielding them from the intense light that would otherwise 

 decompose the forming chlorophyll and otherwise interfere with 

 the vital processes. Red is also a strong absorber of the heat 

 rays in light, and in some cases it may be of service in ensuring a 

 higher temperature in the organs and so expedite their work. 

 Many plants, some tradescantias, hawkweeds and spatter dock, 

 have in their mature leaves layers of cells, frequently the low^er 

 epidermis, filled with a red pigment. So many evergreens assume 

 a brown or reddish hue in the winter owing to the formation of a 

 reddish pigment. However, too much emphasis should not be 

 given to this matter of coloration. The colors of flowers, to be 

 sure, are of service in guiding insects when close at hand, to the 

 proper approach to the flower, and there is probably a significance 

 in the coloration of the algae (p. 173) but in general it is not 

 possible at present to offer an explanation for the variety of colors 

 that characterize so many leaves, especially noticeable in tropical 

 plants, and fruits and other organs, as the radish, and beet. 

 The copper beech grows slower than our green-leaved beech 

 and so doubtless in the case of many plants that have survived 

 not because of their coloration but in spite of it. In many 

 instances the colors are due to the acidity or alkalinity of the 



