CHAPTER VII 



An Analysis of Symbiont Reactions 



The entrance of a microorganism into the tissues of a 

 plant or animal may result in various types and degrees of 

 response. In many instances, there appears to be no 

 response in either the host or the microorganism. In other 

 cases, the host organism responds in both a physiological 

 and morphologic manner. The resistance of the host 

 may be sUght, resulting in "disease," or it may be active 

 and bring about the destruction of the invading micro- 

 organism. There may be a morphologic response in the 

 host, resulting in new tissue formation. When such a 

 tissue production is injurious to the host, we may recognize 

 it as a neoplastic disease (tumors, crown gall, etc.). A 

 catalogue of the known responses in plants and animals to 

 microbic invasion would reveal a multitude of types and de- 

 grees of relationships and reactions. The responses, in 

 general, may be divided into two tj^^es; physiologic and 

 morphologic. Further, these responses may be either 

 temporary or permanent. Our study is particularly con- 

 cerned with those modifications in physiology and mor- 

 phology which may become permanent in a species. It 

 becomes necessary also to refer to some of the temporary 

 responses associated with disease in order to understand 

 more fully the factors involved in Symbionticism. 



The reactions that have been indicated in the relative or 

 host symbiont in the previous chapter were seen to vary in 

 different symbiotic relationships. In the protozoa, it has 

 not been possible to recognize any response, either physio- 

 logic or morphologic. Obviously, if a protozoan animal 

 develops an absolute symbiosis with a bacterial organism, 



