PRODUCTS OF METABOLISM. 1 29 



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 probably not of bacterial origin, but due to chemical oxidation or to 

 the respiration of the living plant tissue. 



PRODUCTION OF 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. 



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 in contact with oxygen again, they produce light immediately, and 

 this sudden flashing is used occasionally by physiologists as a very delicate 

 test for oxygen. The light appears to be produced 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 compound is formed in the cell which gener- 

 ates light when in contact with oxygen. 



The life processes of the photogenic microorganisms are not necessarily 

 connected with the formation of light. Photogenic bacteria are known 

 to lose the power of light production as the chromogenic bacteria may 

 lose the power of pigment production. Phosphorescence has, like pig- 

 mentation also, no bearing upon the development of the cell, and the light- 

 giving compounds may be regarded as incidental waste products. Cer- 

 tain chemical bodies stimulate light production, while others favor the 

 growth only. One of the most important factors in the production of light 

 is sodium chloride. 



