l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



or 10 times their original diameter. This distortion was followed by 

 shrinkage and finally disintegration. Since wave length 2750 A did 

 not affect the cells, Lucas studied the reactions to wave length 2300 A 

 by photographing one specimen with wave length 2750 A, irradiating 

 it for a given time with wave length 2300 A, and then rephotographing 

 with wave length 2750 A. By irradiating and photographing in regu- 

 lar sequence, Lucas obtained a series of photographs showing the 

 cumulative effects of irradiation. At 2300 A, the cells appeared quite 

 opaque, and when the irradiation time was increased for this wave 

 length, separation of the different cell parts occurred. At 2265 A 

 there was also general disintegration of the cells. 



SUMMARY 



The radiotoxic spectral sensitivity of the unicellular green alga 

 Chlorella vulgaris has been determined for 20 wave lengths in the 

 ultraviolet ranging from 2250 A to 3022 A. As a preliminary step to 

 facilitate deductions, an experiment was performed with three relative 

 intensities which demonstrated that for a range of 12-fold the lethal 

 effect as measured by the reciprocal of the reaction time is approxi- 

 mately proportional to the product of the exposure time and intensity. 

 The results are in good accord with earlier observations as far as 

 those went. They show maximum lethal sensitivity at about 2600 A. 

 The wave length 3130 A, slightly longer than the short wave length 

 limit of solar radiation reaching the earth's surface, had no lethal 

 effect on the algal cells although of a greatly higher intensity than the 

 toxic wave lengths. Since it is true that death ensued more quickly 

 in some regions than in others, the radiotoxic virulence, or speed of 

 eft'ectiveness of each lethal ray in killing the algal cells for a radio- 

 toxic quotum has been calculated for 20 wave lengths ranging from 

 2250 A to 3022 A. The results are of small percentage probable error, 

 and they show an interesting wave of virulence alternately waxing 

 and waning with decreasing wave length, but reaching a high 

 maximum at 2323 A. 



LITERATURE CITED 



Brackett, F. S., and McAlistek, K. D. 



1932. A spectrophotometric development for biological and pliotocheniical 

 investigations. Smithsonian Misc. Coll., vol. 87, no. 12, pp. 1-7. 



DUGGAR, B. M., AND HOLLAENDER, Au-.XANDER 



1934. Irradiation of plant viruses and of microorganisms with monochro- 

 matic light. Journ. Bacteriol., vol. 27, pp. 219-256. 



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