THALLOPHYTES 41 



but when a mold fungus attacks bread, it is obtaining food 

 from material produced by living plants, but no longer con- 

 nected with them. Fungi (like the rust) that attack living 

 bodies are called parasites; while those (like the mold) that 

 attack organic material no longer connected with a living 

 body are called saprophytes. 



It must not be thought that parasite and saprophyte are 

 terms of classification. They refer only to two sources of 

 food supply, and there are many Fungi able to obtain food 

 from both sources. Naturally, some Fungi are usually para- 

 sites, and some are usually saprophytes, but they all obtain 

 food from any available source. In fact, many so-called 

 parasites do not attack the living cells of plants, but live 

 in the vessels carrying water (" sap ") and thus choke them. 

 It is convenient, however, in a general way, to distinguish 

 between the parasitic habit and the saprophytic habit, for 

 while the former often brings trouble to living plants and 

 animals, the latter does not. 



The plant or animal attacked by a parasite is called its 

 host, and when the attack interferes with the vigor of the 

 host, the latter is said to be diseased. It is important to 

 understand what is meant by disease, for there is often con- 

 fusion in using the word. For example, rust is often spoken 

 of as a disease of wheat and other cereals, when, in fact, rust 

 is the parasitic fungus that induces the disease. 



The range of attack by parasites is extremely variable. 

 For example, some parasites attack many kinds of plants; 

 others attack only a certain family of plants ; others attack 

 still smaller groups; and still others attack only one kind 

 (species) of plant, and often can select that species with more 

 certainty than does the botanist. Parasites differ also in the 

 amount of the host attacked. For example, some attack 

 the whole plant ; others attack only certain general regions 

 (as shoots or flowers) ; while still others may be restricted 

 to a single kind of organ. 



