30 LEAF COLORATION 



to light is an important factor in controlling their associations 

 and we frequently speak of plants as sun loving, partially so, 

 shade, and deep shade plants. 



The coloration of leaves is often of significance. In addition 

 to the green color, of which we now have some understanding, 

 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 

 At^\j_ early spring vegetation"^ 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. Such a structure would tend to reflect the 

 light and so contribute to the white appearance of the area (p. 37). 

 In some cases these colors may be of service to the plant. The 

 red probably functions as a screen to young organs, shielding 

 them from the intense light that would otherwise decompose the 

 forming chlorophyll and otherwise interfere with the vital proc- 

 esses. 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 tem- 

 perature 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 lower 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. 163) 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 



