320 



BIOLOGY: GENERAL AND MEDICAL 



When this circumstance is carefully analyzed we find 

 that one of the fundamental conditions of true parasit- 

 ism is wanting, for no means is provided for the future. 

 The host dies before the bacilli have become numerous, 

 there is little multiplication of the bacilli in the body 

 after death, and no means is provided by which they 

 shall leave the host to find their way to another. 



The case is quite different with 

 other bacteria. Thus, the bacillus 

 of tuberculosis is unknown in na- 

 ture except in the disease it causes. 

 The microorganisms are eliminated 

 from the body of the diseased in 

 enormous numbers through morbid 

 discharges and find their way to 

 new hosts through the association 

 of the well with the ill. Here there 

 is little doubt of the parasitic nature 

 of the symbiosis. 



In certain cases accident may 

 transform the symbiotic relation- 

 ship from harmless to harmful. 

 Thus the colon bacillus is to be 

 found in nearly all vertebrates, 

 whose alimentary tract it inhabits 

 to feed upon the nutritious con- 

 tents. When local disease of the 

 intestine arises from any cause, in- 

 vasion of the tissues by the bacilli 

 may supervene, and the usually 

 inoffensive organism may occasion 

 disturbances resulting in the death 

 of the host. 



Plant parasites are innumerable 

 and attack animals as well as other 



plants. Bacteria are not infrequently parasitic upon 

 higher plants, an excellent example being found in the 

 wilt disease of cucumbers and melons. The entire class 

 of fungi is composed of organisms that must derive their 



FIG. 111. The ergot of 

 rye, Claviceps purpurea. 

 Ear of rye showing two 

 sclerotia of the fungus. 2/3 

 natural size. (Partly after 

 Tulasne.) (Kerner and 

 Oliver.) 



