PARASITISM AND RESISTANCE 383 



point out that no toxin has ever been demonstrated experimentally. 

 Leach (1919) believes that each physiologic race of Puccinia graminis 

 has its own characteristic food requirements which are met by only a few 

 varieties of the host. According to this hypothesis, if a race of rust 

 enters a host which does not meet its specific nutritional requirements, it 

 dies, and enzymes which are injurious to the host cells are released. This 

 hypothesis is supported by Wellensiek (1927) who worked with Puccinia 

 sorghi. 



While it is evident that the food supply varies with the varieties of the 

 host, it seems equally possible that the difference in nutrients may have a 

 more indirect effect in determining whether the fungus survives. Is it 

 merely that the fungus starves if the host does not provide the appropriate 

 food, or are the conditions in the host unfavorable for the production of 

 certain metabolic products which are essential to the pathogenic actions 

 of the fungus? 



The type of host-parasite relationship found in Phyllachora graminis 

 seems to be unique (Orton, 1924) . This fungus apparently has the power 

 of digesting and absorbing the tissues within the leaf, producing cavities 

 in which the ascocarps later form. The hyphae bore their way through 

 the cell walls of any of these tissues and, in doing so, absorb a portion of 

 the wall. The parenchyma cells become disorganized, and their contents 

 disintegrate. The vascular cells may be invaded and partially absorbed 

 and become filled with hyphae. The most striking physiological charac- 

 teristic of this fungus is its ability to absorb, replace, and engulf the 

 tissues of the host leaf without any external evidence of necrosis of the 

 host. This would seem to indicate the presence of highly active cellulo- 

 iytic enzymes (and perhaps others) confined to the area near the fungus, 

 without the presence of toxic substances, which would cause necrosis of 

 the leaf tissue. 



Actually, comparatively little is known about the activities which 

 lead to parasitism, particularly of the balanced parasites. It is hoped 

 that more planned experiments will be conducted in an attempt to gain 

 more knowledge regarding the mode of parasitism of plant pathogens. 

 Only by understanding the action of the parasite can we understand the 

 basic facts underlying resistance and susceptibility. 



Parasitism and symbiosis with insects. There are numerous reports 

 of the parasitic and symbiotic relations of fungi with insects. For a 

 more complete discussion than this text offers, see Leach (1940) and 

 Steinhaus (1946). In many cases the relationship is solely to the advan- 

 tage of the fungus (true parasitism), but a number of cases of mutualistic 

 symbiosis do exist. The fungi may be disseminated by the insects which 

 serve as their hosts. One can only speculate regarding the basic nutri- 



