12 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



In plotting the curves (see fig. 2), all the points for the final growth 

 ratios were considered. This includes the points found in the set of 

 experiments run with old stimulated cultures as described later. The 

 figures near the points in the curves represent the number of observa- 

 tions entering into the mean. In plotting the curve for 2483 A., cer- 

 tain observations that seemed very wild were rejected in the pre- 

 ferred diagram, drawn with full lines. The dotted curves represent all 

 the observations. 



As shown by studying the curves and the tables, the highest point 

 of stimulation for all the wave lengths is at approximately two-thirds 

 of the intensity of the lethal threshold. The greatest amount of stimu- 

 lation was found by exposure to the wave length 2652 A., and the 

 smallest amount to the wave length 2967 A. These results correspond 

 to the radiotoxic spectral sensitivity of green algae found previously 

 (Meier, 1936). Some stimulation was obtained by shorter exposures 

 than the optimum to each of these four wave lengths. 



To check the lethal threshold points found in the liquid cultures, the 

 following experiment was conducted. Green algal cells growing in 

 Detmer 1/3 were pipetted into the quartz tubes. After irradiation, the 

 cells were pipetted into petri dishes containing sterilized Detmer 1/3 

 agar. They were then placed in the growth chamber and observations 

 were made daily. The results as given below show that the lethal 

 points correspond to those obtained in the liquid cultures. 



