NO. 5 IRRADIATED LETTUCE SEED FLINT AND McALISTER 9 



In connection with the later consideration of inhibitory influences 

 associated with wave lengths characterizing violet, .blue, and green 

 light, it is to be noted that with both solar and Mazda radiation the 

 energy at 7600 A is much greater than at the shorter wave lengths of 

 the visible spectrum. In solar radiation there is a sharp absorption 

 band in the 7600 A region interpreted as due to oxygen in the sun 

 and water vapor in the earth's atmosphere. Notwithstanding this ab- 

 sorption, however, the energy of solar radiation at this wave length 

 region is large. In consequence it would appear that under natural 

 outdoor conditions and under customary indoor experimental con- 

 ditions radiation of a wave length in the long red exerts a relatively 

 powerful inhibitory influence upon the germination of dormant lettuce 

 seed, although this influence is ordinarily more than counteracted by 

 the promoting influence in the yellow-orange-red region. The extent 

 to which the 7600 A region has an analogous effect upon other seeds 

 and upon other phases of light-sensitivity is not known at this time, 

 but because of the high energy and universal occurrence of the radia- 

 tion, its potential significance becomes one of the most intriguing 

 results of its discovery. Further studies of the possible effectiveness 

 of this region in respect to the germination of other seeds and in 

 respect to other phases of light-sensitivity are now in progress. 



An examination of the germination responses to light of the wave 

 lengths indicated in figure 4 reveals that there are two maxima of 

 inhibition in the violet -blue-green region — a major one at 4400 A, and 

 a somewhat subordinate one at 4800 A. It may now be noted that 

 Bachmann and Bergann (i) and Johnston (6), studying the etiolated 

 coleoptiles of Avena sativa Culberson, obtained curves of phototropic 

 sensitivity having two maxima at about these same regions. The two 

 types of data, the one indicating an inhibitory influence of light on the 

 germination of seeds, the other an influence of light on the direction 

 of growth of young etiolated shoots, have been brought together to 

 facilitate comparison in figure 5. 



An examination of figure 5 reveals that within the range of experi- 

 mental error the two types of plant response to light show identical 

 critical wave lengths. Johnston (6, p. 14) and others interpret photo- 

 tropic response as an index of growth retardation on the theory that 

 the light on the exposed side of the shoot inhibits elongation, while 

 on the opposite unexposed side elongation is relatively uninfluenced. 

 The results here reported obviously tend to sustain the correctness of 

 this interpretation. Both of the foregoing types of plant response to 

 light involved etiolated structures, and further studies are in progress 

 relating to the types of plant response characterizing green tissues. 



