130 A TEXT-BOOK OF BOTANY 



supposed that all parasites are harmful to man or even 

 destructive to their host. 



In the case of saprophytes, dead bodies or body products 

 are attacked, and sooner or later all organic matter is 

 attacked and decomposed by them. Were it not for them 

 "the whole surface of the earth would be covered with a 

 thick deposit of the animal and plant remains of the past 

 thousands of years." 



The parasitic and saprophytic habits are not restricted to 

 the Fungi, for they have been developed also by some of 

 the higher plants; but by far the largest display of these 

 habits is that given by the Fungi. It is thought that 

 Fungi have been derived from Algae; that is, that Fungi are 

 simply Algae that have learned the parasitic or saprophytic 

 habit. Some of them resemble certain Algae so closely that 

 the connection seems very plain; but others have become 

 so modified that they have lost all likeness to the Algae. 



No attempt will be made to present even an outline of 

 the classification of this vast and perplexing group. A few 

 illustrations will be selected from the best-known forms, 

 especially those of importance to man. 



77. Bacteria. Bacteria include the smallest known 

 living forms, some of which are spherical cells only 5 ^ 

 inch in diameter. It is estimated that 1,500 of certain 

 rod-shaped forms, placed end to end, would about stretch 

 across the head of an ordinary pin. Even to distinguish 

 ordinary bacteria, therefore, the highest powers of the 

 microscope are necessary; and to study them is too difficult 

 for the untrained student. However, they are so very im- 

 portant to man, on account of their useful and destructive 

 operations, that every student should have some informa- 

 tion about them. Public attention has been drawn to them 

 chiefly on account of the part they play in many infectious 

 diseases, in which connection they are often referred to as 

 "microbes" or "germs." 



