394 PHYSIOLOGY OF THE FUNGI 



(2) The parasite may produce toxic materials or other substances whi( ii 

 may be active at some distance from the fungus, but it usually does not 

 cause the rotting of the tissue. This is illustrated by a number of wilt 

 diseases and by some others. (3) The third type depends upon a con- 

 genial nutritional relationship between the parasite and the host cells. 

 In susceptible hosts of this type there is little or no apparent effect upon 

 the host cells. The resistant hosts may show a high degree of sensitivity 

 to the parasite, which may result in the death of the invaded cells and 

 starvation of the parasite. This type of parasitism is characteristic of 

 the balanced parasites. The balanced parasite enters the susceptible 

 host cell and establishes a compatible food relationship, absorbing the 

 soluble nutrients elaborated by the host, without disturbing the metabolic 

 activity of the host in the early stages. In this respect, the relationship 

 of parasite and susceptible host represents the most highly specialized 

 type of parasitism. 



The destructive parasites, as a rule, are strong producers of toxins 

 and exoenzymes, while the balanced parasites must be quite weak in this 

 respect. In many host-parasite relations studied, there is a change in 

 the permeability of the host cells surrounding the invading hyphae. This 

 is believed to be a direct response to substances secreted by the parasite. 

 Increased permeability would allow greater diffusion of water and nutri- 

 ents from the host cells to the parasite. The metabolic products of the 

 fungi involved in parasitism are for the most part undetermined, but they 

 are known to include toxins, enzymes, and polysaccharides. Since the 

 kind and amounts of such products are known to vary with the composi- 

 tion of the medium in the laboratory, it is believed that like variation 

 may occur in different hosts in nature. 



The basis of resistance to disease may be mechanical, functional, or 

 physiological. Some of the known or proposed causes of physiological 

 resistance are (1) cell-sap acidity; (2) toxic substances of the host; (3) 

 inhibition of the activity of certain enzymes of the parasite by the host ; 

 (4) hypersensitiveness ; (5) incompatible nutritional relationship; (6) 

 decreased permeability of the cell membranes of the host, resulting in 

 partial or complete starvation of the parasite; (7) a combination of 

 various factors acting together. 



The obligate parasites, principally the rusts, offer some challenging 

 unsolved problems for the future students of parasitism. Probably the 

 principal one involves the culturing of such fungi under controlled condi- 

 tions on media of known composition. All of the many attempts to solve 

 this problem have met with failure, yet few investigators doubt that it 

 can be solved. The phenomenon of heteroecism among the rusts is of 

 great interest from the standpoint of food relationships. For instance, 

 sve must either assume that the wheat and the barberry furnish the same 



