WHAT ARE MICROORGANISMS 7 
such plants as molds, mushrooms, toadstools, tree fungi, and hosts 
of others less commonly known. Some of them are of great im- 
portance in farm life, especially as agents in bringing about the 
decomposition of vegetable matter, so that it may be incorporated 
into the soil to be used again; here they play a part secondary 
only to bacteria. They are of endless variety, and it would be 
manifestly impossible here to attempt any consideration of their 
classification. One point concerning them must be understood. 
In all the higher Fungi with which we are concerned the body of 
the plant consists of a mass of delicate threads which grow into 
a dense, usually white mass (Fig. 1). Sometimes the threads are 
a 
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b 
Fic. 1.—One of the higher fungi, the common bread mold, Penicillium glaucum. 
a, the whole plant; 6, one of the spore-bearing branches more highly magnified. 
large enough to be seen easily and sometimes they are so delicate 
that a microscope is required to see the individual threads, though 
the mass of threads may be of considerable size. The mass of 
threads grows on the surface or in the substance upon which the 
plant is feeding. This thread is able 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 is found in all the 
Fungi of this class. So far as the mycelium is concerned, most 
