368 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



our first "outing" of the season. Its lethal action on micro-organisms 

 has been studied by many experimenters, especially in connection with 

 the treatment of disease. The "scare" headlines of the daily press 

 frequently designate it as the "death ray." Ultraviolet radiation 

 covers a wide range of wave lengths. These different wave lengths 

 have their specific characteristics just as truly as the wave lengths 

 in the visible spectrum. 



The Smithsonian Institution has been interested in the specific 

 action of these ultraviolet wave lengths on green algae, one of the 

 lower forms of plant life. Using the variety Chlorella vulgaris^ Dr. 

 Florence E. Meier (1936) has grown cultures on agar plates and 

 exposed them to the ultraviolet spectral lines of a quartz mercury 

 vapor lamp. The intensities of 20 different wave lengths ranging 

 from 2250 A to 3022 A were carefully measured and their effects 

 studied with respect to their lethal sensitivity and to their radiotoxic 

 virulence or speed of effectiveness in killing the cells. 



An algal spectrogram with distinct areas of dead cells is shown in 

 plate 4, figure 2. A photogi-aph of an algal plate exposed to the 

 ultraviolet spectrum has been superimposed on a diagram represent- 

 ing the intensities of the different mercury lines. Three exposures 

 are here shown: (1) 64 minutes, (2) 16 minutes, (3) 32 minutes. 

 The wave lengths are noted across the bottom of the diagram. The 

 heights of the vertical bars represent radiation intensities in thou- 

 sands of ergs/sec./cm ^. It was from plates and data like these that 

 the radiotoxic spectral sensitivity and virulence were calculated. Dr. 

 Meier (1936) reports maximum lethal sensitivity at 2600 A, and a 

 change of virulence with decreasing wave length, which reached a 

 high maximum at 2323 A. 



Let us take a moment to consider a case in the near infrared, just 

 beyond the visible red of the spectrum. In one of our experiments 

 (Johnston, 1932), tomato plants were grown under two sets of wave- 

 length conditions. In one, only visible light was present; in the 

 other, near infrared radiation was added to the visible. Although 

 the near infrared plants were taller and heavier, their appearance 

 was far from normal. A marked decrease in chlorophyll occurred 

 in the leaves and a distinct yellowing and death was noted in some 

 cases. It appears that if this near infrared region is not actually 

 destructive to chlorophyll, it is of little or no benefit to its formation. 



In connection with a discussion of ultraviolet and infrared radia- 

 tion effects, it is interesting to note that in the experiments of Dr. 

 John M. Arthur (1932) at the Boyce Thompson Institute for Plant 

 Research, Inc., on the production of pigment in apples, coloring was 

 increased by ultraviolet radiation, while the near infrared radiation 

 alone or in the presence of visible light had a marked detrimental 



