INFLUENCE OF LIGHT AND OTHER RAYS 225 



been found that molds produce mycelium mostly in the dark, while in 

 daylight sporangia are produced mainly. This difference in the devel- 

 opment during the day and during the night accounts for the concentric 

 rings which are quite commonly found in older mold c6lonies, and 

 which Indicate the age of the culture (Fig. 107). Similar rings are 

 occasionally found with yeast and bacterial colonies, and are possibly 

 due to the same influence of light. 



X-RAYS. — Of other rays, the invisible X-rays and the radium rays 

 have attracted the attention of bacteriologists and physiologists. It 

 is known that the X-rays will destroy living tissue by long exposures; 

 microorganisms cannot be considered less resistant. X-rays are used 

 in the treatment of microbial diseases of the scalp and skin. 



Radium rays are not so well known, and their bactericidal action is 

 doubtful. The treatment of certain bacterial diseases has been 

 attempted, but it has not been appHed as generally as yet as the X-ray 

 method. The sterilization of milk and possibly other foods by this 

 method has been suggested, but the practical appUcation is at present 

 quite improbable because of the cost and the uncertainty of the results. 



16 



