1 88 NUTRITION AND METABOLISM 



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. 



Prodtjction of Light.— The hght-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 quaUty 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 hght, 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 hght outside th6 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 hght. 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 

 pigmentation also, no bearing upon the development of the cell, and the 

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

 Certain chemical bodies stimulate light production, while others favor 

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

 of hght is sodium chloride. 



