8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



badly overexposed when adequate exposure is made for the weaker 

 lines. Exposure 2 lasted 18 hours and shows perceptible lethal effect 

 even in line 3,022 A., which was scarcely noticeable in the color plate. 

 The values of relative lethal effect for the color plate and each of 

 the two exposures on the black plate have been plotted and are shown 

 in Figure i. Those points connected by dotted lines are to be given 

 relatively smaller weight. Of course, it should be emphasized that 

 these measurements are only approximate in that different periods of 

 incubation and different times of exposure may modify the relative 

 effects of different wave lengths. As the lines differ so greatly in 

 intensity it is hoped that further investigation can be undertaken so 

 that the effects of intensity and time exposure may be studied. 



DISCUSSION 



The ultra-violet component of solar radiation at the earth's surface 

 is from the limit of the visible spectrum, 4,000 A. to about 2,950 A. 

 In nature, plants are exposed to invisible radiations in this region. 



The amount of ultra-violet light which the plant receives varies 

 according to the altitude, atmosphere, and season of the year. Life as 

 it is on the earth is possible only because of the ozone formed in the 

 upper layers of the atmosphere by the action of the short wave lengths 

 of the ultra-violet of sunlight on oxygen. This ozone serves as a 

 light filter and thus protects the life on the surface of the earth from 

 the shorter destructive rays. 



Throughout the ages living organisms have probably become adapted 

 to solar radiation as it is received on the earth's surface and very pos- 

 sibly with the same spectrum limit due to ozone. It is, therefore, not 

 surprising that radiation of wave lengths shorter than the solar limit 

 produce unusual effects. While large amounts of ultra-violet of cer- 

 tain wave lengths are lethal, it is possible that very small amounts 

 of the same wave lengths may be not lethal but, on the contrary, stimu- 

 lating to the growth of green algae. With further experimentation 

 we hope to obtain more definite information in regard to the possibility 

 of this stimulative effect. 



SUMMARY 



It is extremely interesting to note that in the regions where the 

 ultra-violet waves beyond 3,022 A., the approximate limit of ultra- 

 violet irradiation in nature, were directed on the culture, the green 

 algal cells were killed. These lethal regions appear as decolorized cells 

 in the green algal plate at the wave lengths 3,022, 2,967, 2,894 2,804, 

 2,753, 2,699, 2,652, and 2,536 A. Wave lengths longer than 3,022 A., 



I 



