528 TEXTBOOK OF BOTANY 



Pasteur's first conviction that microorganisms are the cause and not 

 the result of fermentation came when he was investigating the troubles 

 in tlie beet-sugar distilleries at Lille, France. Besides the normal fer- 

 mentation resulting in alcohol, the vats frequently contained lactic acid, 

 a product for which there was no market. Pasteur found that when 

 yeasts were present alone in the vats, alcohol was formed. When certain 

 rod-shaped bacteria were present, however, lactic acid appeared also. 

 B\ properly transferring the two organisms to sugar solutions he was 

 able to secure either alcohol or lactic acid at will, depending upon 

 whether the veast or the bacterium was present in the solutions. 



The proponents of spontaneous generation were still not convinced 

 and demanded more proof. Pasteur felt certain that bacteria are present 

 in the air and set about to verify his conviction. He prepared numerous 

 flasks of various sterilized media, sealed the sterilized flasks, and later 

 opened them at various places, such as in rural districts, on busv street 

 corners, and on mountain tops. The results indicated clearlv that bac- 

 teria are much more abundant in some areas, such as dustv streets, than 

 in quiet countrysides or mountain tops. Even then the skeptics were 

 unconvinced: they insisted that the sealing of the flasks had brought 

 about an unnatural condition. 



Pasteur soon afterward utilized an experimental idea that did away 

 with the necessitv of sealing the flask. Sterile media were placed in 

 sterile flasks the necks of which had been drawn out into long, narrow 

 S-shaped tubes having verv small openings at the outer ends. In this 

 experiment the flasks were not sealed, and the culture media were given 

 free access to the air. No matter where the flasks were placed or how 

 long thev were kept, bacteria did not develop in the media. But when 

 the flasks were opened so that the media became exposed to air in the 

 usual wav, thev soon became contaminated. The results of these ex- 

 periments were clear, but there was a lack of unanimitv in the infer- 

 ences drawn from them. Pasteur was so sure of the completeness of his 

 demonstration that in a public lecture at the Sorbonne in 1864 he closed 

 his address with these prophetic words: "Never will the doctrine of 

 spontaneous generation recover from the mortal blow of this simple ex- 

 periment." Nearly all thinking people now accept the fact that organisms 

 arise only from preexisting organisms and not by spontaneous genera- 

 tion. Some of the flasks prepared bv Pasteur have been kept to the 

 present time, and no living organisms have appeared in them. 



Two centuries earlier, Redi had proved experimentally that worms 



