PRODUCTS OF MICROBIAL ACTIVITIES 251 



but the best silage is secured by keeping the temperature below 50. 

 Ensilage fermentation is not thoroughly understood, however, and no 

 accurate statements can be made as to the cause of the increase in 

 temperature. Sometimes the temperature in silos does not exceed 

 35. The curing of hay is usually accompanied by a rise of temperature. 

 For some time it was believed that the spontaneous combustion of hay 

 was mainly due to microorganisms, but it has been shown recently 

 that even sterile hay will show a rise of temperature under certain 

 conditions. This does not exclude the formation of heat in hay by 

 microorganisms under other circumstances. The heating of tobacco, 

 of green or moist grain or corn is not of bacterial origin, but due to 

 the respiration of the living plant-tissue. 



PRODUCTION or LIGHT. The light-producing or photogenic organ- 

 isms are quite numerous and occur more frequently than is generally 

 believed. The phosphorescence of decaying tree stumps and leaves in 

 the woods and of meat and fish in the cellar are well-known phenomena. 

 The phosphorescence of wood and leaves is generally caused by Hypho- 

 mycetes; certain mushrooms have this quality in a very high degree. 

 The light of meat and fish is usually generated by bacteria, of which at 

 least twenty-six species have been described. 



The most obvious evidence of liberation of energy in the physiology 

 of protozoa is seen in their movement. Certain protozoa, Noctiluca 

 for example, however, emit light and produce the phosphorescence 

 often observed in sea water. From analogy with higher animals it is 

 to be supposed that heat and electrical changes are also produced." 



Many experiments have been carried on in order to discover the 

 nature and origin of the light, but, so far, few results have been obtained. 

 The phosphorescence is due to an oxidation process; all photogenic 

 organisms cease to generate light when the oxygen is removed. As 

 soon as they come into contact with oxygen again, they produce light 

 immediately, and this sudden flashing is used occasionally by physiolo- 

 gists as a very delicate test for oxygen. The light appears to be pro- 

 duced always within the cell; no cell product has ever been found to 

 give rise to light outside the cell. It is possible that a chemical com- 

 pound is formed in the cell which generates light when in contact 

 with oxygen. 



The life processes of the photogenic microorganisms are not neces- 

 sarily connected with the formation of light. Photogenic bacteria are 



