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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



When this point was determined a specially constructed thermo- 

 couple replaced the seedling, and by means of a galvanometer the two 

 light intensities were measured. 



From a number of such experiments the curve shown in figure 6 

 was constructed. This curve illustrates the sensitivity of the oat 

 seedling (plotted vertically) to the wave lengths of light (plotted 

 horizontally). The sensitivity increases rapidly from 4100 A to 

 4400 A, then falls off somewhat to about 4575 A, and again rises to 

 a secondary maximum at about 4750 A. From this point the sensi- 



4000 4100 4200 4300 440O 4300 460O 4100 4600 4S00 SOOO SIOO S200, &30O 



Figure 6. — Phototropic sensitivity curve of oat coleoptile. The onlinates are relative 

 sensitivity values, the abscissae wave lengths in angstroms, and the horizontal bars 

 indicate the wave-length ranges of the balance points. 



tivity decreases rapidly to 5000 A, from which point it gradually 

 tapers to 5461 A, the threshold of sensitivity on the long-wave-length 

 side. Briefly, it may be concluded that the region of greatest sensi- 

 tivity is in the blue. That is, growth is retarded most by blue light. 

 Orange and red light have no effect in retarding the growth of these 

 oat seedlings. 



An interesting phenomenon closely paralleling phototropism has 

 been observed for a certain type of seed germination. Dr. Lewis H. 

 Flint, of the United States Department of Agriculture, found that 

 the short wave lengths of light — violet, blue, and green — inhibited the 

 germination of light-sensitive lettuce seed, and that the long wave 



