SUNLIGHT UPON MICROORGANISMS 209 



3. Duplicate with the B. typhosus culture, placing the 

 pipettes immediately after using in 1-1,000 HgCb. 



4. Cut any design out of black paper and paste on the 

 bottom of the Petri dish. 



5. Place the dish bottom side up in direct sunlight for 

 two hours. 



6. Set the dish away in the dark at room tempera- 

 ture. Observe the growth and explain. Which organism 

 is the more sensitive to sunlight? Conclusions? 



Note. Heat B. typhosus plate 1 hour in steam before cleaning the 

 Petri dish! 



7. What theories have been advanced as to the mechan- 

 ism of destruction by direct sunlight? Does sunlight have 

 any effect on bacterial spores? 



How are other forms of organisms affected by light? 



What is phototaxis? Do bacteria ever exhibit this 

 phenomenon? 



Which portion of the spectrum is most active? 



What relation does the wave-length of light rays bear 

 to the activity of the rays? 



How do diffuse light, electric or other forms of artificial 

 light, X-rays, radium rays, etc. compare with direct sun- 

 light as to their action on bacteria in general? 



8. Give all data and state results in full. Draw any 

 conclusions that follow and point out any practical applica- 

 tions. 



REFERENCES 



MARSHALL, C. E.: Microbiology, pp. 162-163. 



SMITH, ERWIN F. : Bacteria in Relation to Plant Diseases, Vol. I, 



p. 71; Vol. II, p. 324. 

 LAFAR, F,; Technical Mycology, Vol. I, pp, 60-63, 



