6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



RESULTS 



The results of irradiating the 10 plates of the first series with 

 ultraviolet rays are shown in tahle i. The regions of decolorized cells 

 that appeared in the green plates where the wave lengths of ultra- 

 violet proved to be lethal or radiotoxic are tabulated with the initial 

 dates of their appearances. The experiment was brought to an end 

 (3ctober 30, an arbitrary date, 2 months after the irradiation date, 

 but the plates remained in good condition until November 30 and 

 showed no further marked differences in appearance. The total 

 number of radiotoxic regions for each ex]^osure was listed at this 

 time. See plates 2 and 3. 



The results of the second exi)eriment, in which i 1 plates including 

 the uninoculated agar plates were irradiated, are given in table 4. The 

 radiotoxicity as shown by the colorless algal regions which were 

 present on the jilatcs June 24. 1933, 2 months after the irradiation 

 date, is indicated here with the exposure times. (See pi. i.) 



As indicated in tables i and 2, the inoculations were made from 

 2 to 5 months ])revious to the dates of irradiation. This dift'erence 

 in the age of the cultures had no apparent eft'ect on the response 

 of the algae to the ultraviolet irradiation. 



THE LETHAL RADIOTOXIC THRESHOLD 



A study of tables i and 4 shows that the lethal radiotoxic threshold, 

 or minimum amount of radiotoxicity required to produce lethal effect, 

 for wave lengths 2652 and 2804 A lies between 100 and 120 seconds 

 and probably midway between 105 and 120 seconds (the lOO-second 

 exposure being with light of greater intensity) for intensities 1,960 

 and 1,840 ergs/sec. cm- respectively. If it is assumed that the radio- 

 toxic effect is proportional to the intensity and the duration of ir- 

 radiation, then for 1,000 ergs/sec. cm- the exposures required for 

 2652 and 2804 A may be set as i .96 x 1 1 2 = 220 seconds and i .84 x 

 1 12 = 206 seconds respectively. For 2699 and 2753 A, 8 minutes or 480 

 seconds did not always suffice for killing the cells but usually did, so 

 it is near the threshold. Then for 1,000 ergs/sec. cm-, the required 

 time would be 0.64x480 = 307 seconds and 0.54x480 = 259 seconds. 

 Also, 2894 and 2967 A occur once or twice at 480 seconds. Hence 

 for 1,000 ergs/sec. cnr, the required time would be 0.93x480 = 446 

 seconds and 2.45x480=1,176 seconds. Also, injury at 2925 and 

 3022 A first appears at 1,920 seconds and then not always, therefore 

 similarly we find required times of 0.44x1,920=845 seconds and 



