THE FUNGI 263 
of alge. Habits of parasitism, whether among animals or 
plants, very frequently cause them to become less perfectly 
developed than their nearest non-parasitic relatives. 
The economic importance of some fungi has been already 
mentioned (Sects. 311, 3138, 316, 326, 335). No sweeping 
statement can be made that fungi are generally useful or 
generally injurious. They benefit man not only by directly 
furnishing a few articles of food, but also by helping to 
destroy dead animal and plant matter. This would remain 
forever without decaying if it were not for saprophytic 
fungi, especially bacteria, and thus the available raw mate- 
rial for making plant food would soon be exhausted and 
all life and growth cease. Certain bacteria which live on 
the roots of plants of the Pea family serve to convert the 
nitrogen of the goil-air into nitric acid available for use 
by the plant to which the bacteria are attached. Vast 
amounts of food for animals are thus produced. 
Fungi injure man by causing diseases in useful, wild, 
and cultivated plants, in domestic animals, and in human 
beings. The importance of bacterial diseases may be 
partly understood from one striking instance. The Black 
Death of the fourteenth century is considered to have 
been due largely to the attacks of the bacillus, which 
causes the bubonic plague. That single epidemic caused 
the death of about twenty-five million people. 
340. Additional Notes on the Fungi.— Only a few species 
of fungi, out of some forty thousand that are known, have 
been outlined for laboratory work in the present chapter. 
The student can, however, hardly fail to learn from these 
studies something of the extreme diversity of fungi in 
almost every respect except the one general characteristic, 
‘inability to live on inorganic material, that is, to do the 
