WHAT ARE MICROORGANISMS? 
in many cases, by growing, to force its way into the solid mass 
of hard substances, like wood, and to push itself between the wood 
fibers. Thus it is a primary agent in effecting the destruction of 
wood. Such a mass of branching threads is called a mycelium, and 
FIG. i. One of the higher fungi, the common bread mold, Penicillium glaucum. 
a, the whole plant; b, one of the spore-bearing branches more highly magnified. 
is found in all the Fungi of this class. So far as the mycelium is 
concerned, most of these plants are much alike. But the different 
species have many different methods of reproduction, and it is 
chiefly upon their reproductive bodies that botanists rely to distin- 
FIG. 2. Mucor, a common mold, 
showing mycelium and spore for- 
mation. 
FIG. 3. Aspergillus, a common mold, show- 
ing mycelium and spore formation. 
guish the different species. After the mycelium has grown for a little 
time, it commonly sends up into the air small or large branches that 
produce spores, or reproductive bodies. The method of spore 
production differs sufficiently in the different fungi to make it 
