Photoperiodism and Light Quality • 31 



complex organic compounds, although many inorganic salts are 

 highly colored. The absorption spectrum of a given pigment, by 

 which is meant a curve indicating the relative degree to which it 

 absorbs various wavelengths of light, is characteristic of that com- 

 pound alone, or at least of a small class of similar substances. Thus 



Fig. 3-1. Method of holding single leaves (these are soybean leaflets) in the 

 image plane of a spectrograph for subsequent irradiation with various wave- 

 lengths of light. (Photograph from Hendricks and Borthwick, Proc. First Int. 

 Photobiol. Cong. [1954], courtesy of Dr. H. A. Borthwick, U. S. Department of 

 Agriculture.) 



the action spectrum for any process affected by light— a curve 

 indicating the relative effectiveness of different wavelengths on the 

 process— may provide information as to the nature of the com- 

 pound or compounds by which the light is absorbed. For example, 

 part of the evidence for the role of chlorophyll in photosynthesis 

 is the observation that the light most active in that process— blue, 



