l6 BACTERIA, YEASTS, AND MOLDS 



Structure of Molds 



It requires microscopic study to make out the structure 

 of molds, but it is important to understand this structure in 

 order to be able to explain the conditions under which they 

 grow. If we study a young mold before it has begun to 

 produce its fruit, it is found to consist of a long, highly 

 branching thread (Fig. 5). When it begins to grow all 

 that can be seen is this tangled mass of delicate threads. 

 The threads are so minute, as a rule, that the individual 

 fibers are only just large enough to be seen by the naked 

 eye, and in many cases they are too small to be seen 

 except with a lens. The thread of the blue mold is too 

 small to be seen without a microscope. The threads are 

 practically always of a whitish color, nearly transparent 

 when examined under the microscope, and appear as 

 shown in the several figures. In some species of molds 

 they grow into a very dense, felty, rather tough mass. 

 In other species they form a loose mass of coarser fibers 

 (Fig. 2). 



An important point to be remembered is that these, 

 threads, by their growth, can penetrate into the depths of 

 the material upon which they are growing. If they are 

 upon the surface of bread, the fine fibers push their way 

 down into the substance of the bread. If they grow upon 

 cheese, the threads force their way into the body of the 

 cheese. When growing upon any soft food material, the 

 mold threads, though visible only on the surface, really 

 extend into the substance for a considerable distance, 

 although they are so small and transparent that we cannot 

 follow them. Of course the readiness with which a mold 



