6 THE GENERAL CHARACTERS OF MICROORGANISMS 
might properly be called microérganisms. On the other hand; 
some plants of large size, like molds and toadstools, have a part 
to play in producing the decomposition of organic structures very 
similar to that played by bacteria. These cannot properly be 
called microdrganisms, but nevertheless they must be included 
with the study of bacteria and yeasts, since they perform similar 
or closely allied functions in nature. A better term to cover the 
organisms which we must study might be the rather broad term 
of Fungi, for all of the organisms with which we are concerned 
are ordinarily placed in this general class of plants. But this 
term is also unsatisfactory, since it fails to convey the idea that 
the organisms are largely microscopic. ‘To most people the term 
fungus gives at once the impression of a large plant; and we are 
chiefly concerned with microscopic forms. We shall, therefore, 
still use the term microdrganism, 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 con- 
cerned are generally grouped together by botanists in the class 
called Fungi. There is one characteristic common to all Fungi— 
they all lack green coloring maiter. 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 nourish- 
ment from the mineral world. The Fungi, since they are all color- 
less, must live upon food furnished them by other plants or 
animals. It is this fact that gives them their significance in 
nature, and explains their important relations to farm life. Fungi 
are very abundant everywhere, and there are thousands of dif- 
ferent kinds. For the purpose of our study, we may recognize 
three groups: 
1. Higher Fungi—Under this general name we will include a 
large number of colorless plants, mostly of large size. It includes 
