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 



