176 MICRO-ORGANISMS AND FERMENTATION. 



in connection with any one particular species will vary 

 according to the nature of the food supply. 



In general, it may be remarked that organisms can with- 

 stand temperatures below the minimum without permanent 

 injury. Thus, according to experiments by Schumacher, 

 yeast cooled down to 113 C. was not killed. Macfadyen 

 subjected bacteria to gradual cooling down to 172, and 

 even to 190 C., for a period of twenty hours, and mould 

 spores to 210 C. without killing them. On the other 

 hand, a slight increase in temperature above the maximum 

 is often fatal. It is only spores of bacteria, particularly when 

 dry, that are resistant to high temperatures. Many spores 

 may, under these conditions, be heated for a short time to 

 140 C. without injury. 



It is an interesting fact that some species of bacteria 

 capable of thriving at a temperature of C. exist, and others, 

 as stated, develop best at 50 C., or even 70 C. 1 



Light is of vital importance for the fungi, although not 

 to the same degree as is the case with green plants. The 

 injurious effect which light has on the growth of bacteria, for 

 instance, is now widely recognised, and it has been ascertained 

 that for fungi in general both the nature of the nutritive 

 medium and the temperature have a regulating effect on the 

 action of light. From the physiological standpoint, the most 

 active rays of light are the blue, violet, and ultra-violet rays. 



The natural purification of rivers is generally accepted 

 to be due to the germicidal effect of light. 



Kny found that subdued light has no influence on yeast. 

 Lehmann, who used the intensive light of an arc lamp, but 

 experimented at low temperatures, arrived at the same result. 

 He found that light had a retarding influence on the 

 multiplication of yeast cells at 18 C. and above, and that 

 these were killed by prolonged exposure to direct sunlight, 

 whilst diffused daylight delayed the process of budding. 



Many moulds can endure sunlight without injury, but 

 intense illumination frequently restricts the longitudinal 

 growth of the mycelial threads. Some species produce only 

 mycelia in the dark, the reproductive parts requiring light 

 for their development. In the case of Botrytis cinerea, the 



