SUN RAYS AND PLANT LIFE— JOHNSTON 



367 



lengths— yellow, orange, and red— promoted germination. So inter- 

 esting did these observations prove that he and Dr. E. D. McAlister, 

 of the Smithsonian Institution, carried out a much more elaborate 

 and detailed experiment (Flint and McAlister, 1935, and Flint, 1936), 

 Seeds exposed to red light sufficient to bring about a 50 percent germi- 

 nation had superimposed upon them the prismatic spectrum of a 

 Mazda light. The resultant germination, as influenced by different 

 wave lengths, is shown in the form of a curve in figure 7. 



Had the seeds not been exposed to the spectrum, their germination 

 would luive been 50 percent as represented by the horizontal dash 



100 



3000 



4000 



5000 



6000 



70O0 



8000 



Figure 7. — Percentaf^e germination (ordinatts) of ligiit-sensitive lettuce seed in different 

 wave-length regions (abscissae) of the spectrum after an exposure to red light sufficient 

 to effect a 50-percent germination. (Courtesy Flint and McAllister.) 



line. The germination of seeds exposed to wave lengths lying ap- 

 proximately between 4000 and 5200 A was inhibited. That between 

 5200 and 7000 was greatly promoted. An interesting and heretofore 

 unobserved phenomenon was found in the red at about 7600 A. Here 

 also germination was inhibited. Although this inhibitory region 

 in the red has not been detected in our phototropic responses, it may 

 have been overshadowed by other effects not yet properly isolated. 



Experimentation has clearly demonstrated enormous differences in 

 response of living plant tissues to different wave lengths of radiant 

 energy in the visible spectrum. When such interesting reactions 

 occur in visible light, one becomes curious as to what effects are found 

 with wave lengths shorter than the visible violet and with those 

 longer than the visible red. Time will not permit giving more than 

 a single example in each of these two regions. 



The harmful action of ultraviolet radiation is familiar to all; its 

 painful action has been felt by most of us at the bathing beach after 



