SYMBIOSIS. 



257 



C. Parasitism. 





369. I. Fungi. — A very large number of colorless plants 

 have adapted themselves to live upon living plants or ani- 

 mals which they 

 force to act as their 

 unwilling hosts. By 

 the presence of the 

 parasite the normal 

 functions of the host 

 or its normal growth 

 or both are more or 

 less seriously inter- 

 fered with, so as to 

 produce disease, 

 slight or grave, local 

 or general, accord- 

 ing to the circum- 

 stances. Many ani- 

 mals are thus preyed 

 upon by bacteria 

 and fungi. Most communicable diseases, such as typhoid 

 fever, diphtheria, and tuberculosis, are 

 known to be due to the transfer of the 

 parasite from the diseased individual to the 

 healthy one. In a similar way bacteria live 

 as parasites upon green plants, causing 

 disease and often death. The number of 

 Fig. 216 — Hyphs of bacterial diseases among plants is relatively 



a lichen, Cladoniii . i r 1 • i 



/»?-ra!f<z (see fig. 36), small, for comparatively few bacteria nave 



enveloping an alga, .... 



Proiococcus. Mag- been able to adapt themselves to living in 



nified 950 diam. — . 



After Kerner. the acid Cell sap of plants. The number of 



diseases of plants due to parasitic fungi, on the contrary, 



Ftg. 215. — A lichen {Parmelia co-nspersa) growing on 

 a stone, showing the leaf-like thallus (mycelium), with 

 many cup-like fructiiications. Natural size. — After 

 Frank. 



