232 



BIOLOGY 



of cattle, producing serious and fatal brain disease. Malarial 

 organisms (Fig. 25) live as parasites in human blood and pro- 

 duce malaria. Various parasitic bacteria produce serious dis- 

 eases in man, as typhoid fever, tuberculosis, diphtheria, etc. 

 The same is true of plants. The various wiltSj rusts, and blights 

 are serious plant diseases, frequently spreading from plant to 

 plant, and producing death and destruction of the host. All 

 are produced by parasites growing in the plant tissues. Fungi 

 of various kinds are the cause of the greater number of plant 



FIG. 109. A WILLOW 

 LEAF ATTACKED BY 

 "MILDEW" CAUSED 

 BY A PARASITIC 



FUNGUS 



Fia. 110. THE "BITTER ROT"" 



OF CURRANTS 



Produced by the parasitic fungus 

 (Gleosporiurri) . Most of the currants 

 have dropped from the stem and the 

 rest are rotted. 



diseases; Figs. 109 and 110. In other cases, the effect upoii 

 the host is far less serious. Some parasites may live upon a 

 host without seriously affecting it. For example, a number of 

 bacteria live in our intestines; they may be called parasitic, since 

 they dwell within a living host; but instead of being injurious, 



