Light source*. 



5000 



6000 



Spectrum band 



Figure 1. — In this diagrammatic drawing, room length has been much foreshortened to f»rmit showing 

 some details of the large ARS spectrograph and the relationship of the instrument to plants irradiated 

 by it. In the initial experiments that showed the importance of red, each soybean test plant when 

 ready to flower had all foliage removed except one leaf. This leaf was fastened to a screen for brief 

 irradiation during each night photoperiod--to record flowering response to separate colors of spectrum 

 light. 



would have maximum effect, the scientists each night set up group after 

 group of plants to be irradiated for different test times from seconds 

 to a half -hour. 



A red light interruption of even 30 seconds caused soybean and 

 cocklebur plants to start clocking all over when dark was restored. 

 This was evident because these plants never registered enough darkness 

 to signal flowering time when the test conditions were continued for many 

 weeks. 



Plants receiving an interruption of other colors of light either flowered 

 as if no light had appeared, or were affected at most only slightly. 



When the outstanding effectiveness of red was confirmed with varied 

 kinds of plants, the region of the spectrum ruling photoperiodism was 

 mapped in the wave area from about 5800 A. to about 7200 A. 



Clue to an Unknown Chemical 



These spectrum tests gave scientists the first strong clue to the 

 plant's responding mechanism. They had learned that both long -day and 

 short -day plants respond to the same wave lengths of light. This could 

 only nnean that all of these plants, so different in photoperiod require- 

 ments, nnust contain the same responding nnechanism. There must be in 

 plants some unknown chemical substance strongly influenced by the red 

 in sunlight. 



