4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



Lindner (1922) also irradiated yeast with ultraviolet and found that 

 it increased the speed of fermentation. He reports that 20 to 30 per- 

 cent of the cells in the irradiated culture died, thus indicating that 

 he had given the cells a sublethal dose of ultraviolet. 



Owen (1933) reports increased fermentative power of yeast caused 

 by irradiation with a carbon arc, which is rich in wave lengths 2300 

 to 3100 A. 



Hollaender and Curtis (1935), on irradiating colonies of bacteria, 

 Escherichia coli, with monochromatic ultraviolet radiation below 

 3000 A. with exposures that killed some of the organisms but not all 

 of them, found that growth of the surviving irradiated bacteria was 

 retarded as compared with the control (nonirradiated) bacteria. But 

 when the bacteria had completed their growth, the same number of 

 organisms was present in both the irradiated and the control suspen- 

 sions. The irradiated culture increased in number quite rapidly in 

 the early part of the "lag phase," then slowed down, thus suggesting 

 stimulation. Tests showed that the increased growth of the irradiated 

 bacteria was not produced by the decomposition products of the dead 

 bacteria or by the excretion of any substance by the irradiated or- 

 ganisms. The authors thought that stimulation was suggested, but 

 the possibility of recovery of the irradiated bacteria should not be 

 entirely excluded. 



Hollaender and Duggar ( 1937) , in their treatment of yeast and bac- 

 teria with sublethal doses of 2650 A., observed that the survivors of 

 the irradiation proliferated when transferred to salt solutions in which 

 the controls died. 



Loofbourow and others (1938, 1939), from their intensive studies 

 on the stimulating effect of ultraviolet and other lethal agents in sub- 

 lethal quantities on cells, believe that the effect is due to the production 

 by injured living cells of "wound hormones," released as a physiologic 

 response to injury into the intercellular medium. Their work is on 

 embryo chicks and mouse tissues, yeast, and bacteria. 



EXPERIMENTAL PROCEDURE 



In the experiments on the lethal effect of the ultraviolet rays, the 

 alga Chlorclla vulgaris was found to be well adapted to the research 

 because of the uniformity and speed with which the single oval cells 

 covered the surface of the agar plates used. For experimentation in- 

 volving direct counting, this alga is highly unsatisfactory owing to the 

 minute and irregular size of the cells, which vary from 3 to 5 //,, and 

 to the fact that the cell multiplies by oval or elliptical spores, which 

 may range in number from two to four. 



