THE FUNGI 263 



of algse. 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, 313, 316, 326, 336). 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 soil-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 



