THE USEFUL MICROBES 
51 
then, in materials that at other times are too 
dry for their life. They also grow best in places 
where the air is very quiet. Any wind keeps 
them from flourishing, probably because an air 
current drys up the moisture too quickly from 
the surface of the material on which they are 
growing. While they grow in both light and 
darkness, they grow better in the dark than in 
the light. The action of direct sunlight hurts 
them, and usually they will not grow on a 
surface exposed directly to the sun. They also 
grow best in warm places. A temperature near 
freezing prevents their growth. 
Because they grow all through the food on 
which they live, and absorb part of it for their 
own food, they cause changes in its appearance, 
taste, and odor, which usually ruin it. How¬ 
ever, in cheese it is the changes that molds 
produce, and the taste and odor they create 
that are desired. So the action of mold does 
not always make food unwholesome, though 
in other foods than cheese, we usually do not 
like the new taste and odor; so do not eat them. 
Where do 
molds grow? 
