6 THE GENERAL CHARACTERS OF MICROORGANISMS. 
once the impression of a large plant; and we are chiefly concerned 
with microscopic forms. We shall, therefore, still use the term 
microorganism, although some of the plants that we shall refer to 
are not microscopic. In this discussion we are concerned chiefly 
with Bacteria, secondarily with Yeasts, and to a less degree with 
the Higher Fungi. 
The Fungi. All of the plants with which we are here concerned 
belong to the class that botanists call Fungi. There is one charac- 
teristic common to all Fungi they all lack green coloring matter. 
This green coloring material in ordinary plants makes it possible for 
them to live upon the mineral ingredients in the soil; and green 
plants only can be thus nourished. The colorless plants are 
unable to obtain nourishment from the mineral world. The Fungi, 
since they are all colorless, must live upon food furnished them by 
other plants or animals. It is this fact that gives them their signifi- 
cance in nature, and explains their important relations to farm life. 
Fungi are very abundant everywhere, and there are thousands of 
different kinds. For the purpose of our study, we may recognize 
three groups: 
i. Higher Fungi. Under this general name we will include a 
large number of colorless plants, mostly of large size. It includes 
such plants as molds, mushrooms, toadstools, tree fungi, and hosts of 
others less commonly known. Some of them are of great importance 
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 con- 
cerning 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. i). 
Sometimes the threads are large enough to be seen easily and some- 
times they are so delicate that a microscope is required to see the 
individual threads, though the mass of threads may be of consider- 
able size. The mass of threads grows on the surface or in the 
substance upon which the plant is feeding. This thread is able 
