36 BACTERIA, YEASTS, AND MOLDS 



Darkness. Molds will grow both in light and darkness, 

 but on the whole they grow somewhat better in darkness 

 than in light. Indeed, the action of direct sunlight is 

 injurious to them, and most species of molds fail to grow 

 upon the surfaces of material exposed to sunlight. As a 

 result of this we rarely find molds growing upon the free 

 surfaces of materials exposed to the sunlight or even to 

 bright light. This is not universally true, but it is the 

 common experience of housewives to find that materials, 

 when shut up in dark closets, are very much more liable 

 to mold than when left in a light room. This is doubtless 

 due, in part, to the fact that exposure to sunlight, or even 

 to the air of a light room, evaporates moisture rapidly 

 and thus checks molding. But it is not wholly due to 

 this, for light itself appears to be deleterious to mold 

 growth. 



Temperature. Molds require a moderately warm tem- 

 perature for vigorous growth. At a temperature slightly 

 below freezing they will not grow at all, and at a few 

 degrees above freezing their growth is very slight. Some 

 species of molds, however, grow readily enough at 40°, 

 growing better at this than at a warmer temperature. 

 Hence it follows that the temperature of an ice chest will 

 not wholly prevent molding. Most common molds, how- 

 ever, either fail to grow at an ice-chest temperature or 

 grow very slowly. As the temperature increases, how- 

 ever, the growth becomes more vigorous, and at tem- 

 peratures varying from 70° to 100° the growth of these 

 plants is stimulated to their highest activity. A practical 

 result from these facts is that any material which can be 

 kept sufficiently cool will fail to show signs of mold, even 



