SIGNIFICANCE OF ACTION SPECTRA 



43 



be possible to gain information on both efficiency ((f)/n) and absorp- 

 tion A as in the following case from experiments published only in ab- 

 stract.^ 



Yeast cells were exposed in suspension to a suitable range of ultra- 

 violet energies at representative wavelengths. These were plated out 

 and colonies were counted. Thus a family of log survival curves 

 could be constructed for different suspension densities as well as wave- 

 lengths. From each curve a 50% survival point is obtained and used as 

 our action end point. These end point values of ht/N are plotted in 

 Fig. 1 for different choices of cell suspension density. At the left near 



4 5 6 7 8 9 10 



NUMBER PER CC X 10"" 



Fig. 1. Energy of ultraviolet incident for 50% survival of a rapidly 

 stirred cell suspension plotted against the population density. Each point 

 is found from a survival curve not shown. 



2 X 10^ cells/cc we have the points for a typical "thin sample" action 

 spectrum. Plotting the reciprocal, N/Iot, against wavelength, we have 

 curve B, Fig. 2. Here the greater part of the energy is lost through the 

 cuvette, i.e., the transmission T is large. Again from Fig. 1, as the 

 density of cell suspension is increased, this loss diminishes and a mini- 

 mum value is reached. To some extent, treated cells screen untreated 

 cells so that further increase in density reduces the efficiency. The 



2 F. S. Brackett and A. Hollaender, Physical factors governing biological 

 action of radiation. Phys. Rev., 55 (1939). Abstract. 



