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 Alge; that is, that Fungi are 
simply Alge that have learned the parasitic or saprophytic 
habit. Some of them resemble certain Alge so closely that 
the connection seems very plain; but others have become 
so modified that they have lost all likeness to the Alge. 
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 z45 
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.” 
