2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



The nutritive solution in which the algae were grown is Detmer ■§, 

 a modified Knop solution, made up in the following proportions and 

 then diluted to one-third : 



Calcium nitrate i . gram 



Potassium chloride 0.25 " 



Magnesium sulfate 0.25 " 



Potassium acid phosphate 0.25 " 



Ferric chloride 0.002 " 



Distilled water i. liter 



The surface of a glass plate of dimensions 8 x 10 cm was ground 

 so as to retain the agar poured on it. This plate was placed in a large 

 petri dish 15 cm in diameter and after sterilization in the autoclave 

 at 15 pounds pressure for 20 minutes was covered with a layer about 

 4 mm thick of Detmer i agar 2 percent and then sterilized a second 

 time. 



When the agar had gelled, a heavy suspension of green cells of 

 Chlorella vulgaris that had been growing in an Erlenmeyer flask of 

 Detmer ^ solution in diffuse light from a north window was poured 

 over the agar in the petri dish. This suspension was allowed to 

 remain on the agar for 24 hours ; then the excess was removed. The 

 petri-dish culture was placed in diffuse light from a north window 

 until the surface of the agar was covered with a uniform green growth 

 of algal cells. The glass plate covered with agar and algae was then 

 cut out of the surrounding agar in the petri dish and placed upright 

 in a sterilized closed brass container with a quartz window. A decker 

 was arranged in front of the slit of the spectrograph to permit the 

 exposure of three different portions of the plate for three different 

 time periods. 



Previous experimentation (Meier, 1934) has demonstrated that the 

 wave lengths that prove lethal to the green cells of this alga do not 

 affect the culture medium which covers the glass plate in any way that 

 will accelerate or retard the subsequent growth of the algal cells. 



The agar plates used in the experiments described here .were inocu- 

 lated September 6, 1934, and irradiated in October and November, as 

 noted in table i. Those plates irradiated in December were inoculated 

 November 7. Previous experimentation (Meier, 1934) has shown 

 that the difference in the age of the cultures has no apparent effect 

 on the response of this alga to the ultraviolet irradiation. 



A quartz mercury arc was used for irradiation of these cultures. 

 The method of making the absolute measurements of the intensity 

 of the lines in the ultraviolet spectrum with the vacuum thermocouple 



