382 PHYSIOLOGY OF THE FUNGI 



translocation of food, transpiration, respiration, and photosynthesis. 

 Increased respiration has been reported for some hosts, while a decrease 

 has been found for others. The rate of transpiration is usually increased. 

 An early infection of orange rust on Ruhus may even cause the formation 

 of stomata on the upper epidermis, where they are normally lacking 

 (Dodge, 1923). 



The reactions of the chloroplasts of the host cells are believed to indicate 

 the degree of adjustment between the host and parasite (Rice, 1935). 

 Local chlorosis and streaking are common symptoms of a number of 

 diseases caused by haustoria-forming parasites. 



Thatcher (1939, 1942) has shown that certain obligate parasites cause 

 an increase in permeability of the cell membranes of susceptible hosts. 

 There was a decided reduction in osmotic pressure of the tissues of Pisum 

 surrounding the rust hyphae. If the fungus is unable to bring about an 

 increase in permeability so that it can obtain its necessary nutrients, the 

 host is resistant. Thatcher found evidence that the plasma membranes 

 of some resistant varieties of wheat may actually become less permeable 

 as a reaction to the rust hyphae, and starvation of the fungus may result. 



The change in permeability incited by the balanced parasites seems 

 to be similar to the action of the destructive parasites, except for the 

 matter of degree. Thatcher (1939) believes that parasitism in the rusts 

 has become highly specialized, and the intensity of the effect on permea- 

 bility of the cell membranes has been reduced. The substance involved 

 is apparently a metabolic product of the fungus. 



If the conditions afforded by a certain variety of host are favorable 

 for the production of a comparatively large amount of toxin (assuming 

 that this substance is a toxin) , the host cells may be killed and the further 

 development of the obligate parasite would be prevented. The sudden 

 death of the host cells is the condition described by Stakman (1914) as 

 hyper sensitiveness. Hypersensitive hosts are highly resistant or immune 

 to the pathogenic action of the obligate parasites. Stakman reported 

 that, in varieties of wheat resistant to Pucciiiia graminis tritici, when 

 the hyphae of the fungus come in contact with the host cells, the latter 

 often show plasmolysis, disintegration, and finally death. After the 

 death of a few surrounding cells the tips of the hyphae die. However, it 

 was discovered that in some cases the hyphae may die before the host cells 

 are killed. Stakman concluded that the problem of resistance to rvists 

 is one of toxins of the parasite or the host, or both, and can best 

 be explained by what he terms the "toxin or enzyme theory." Brooks 

 (1948) also concluded that the death of the parasite is due to the lethal 

 action of the host rather than to starvation. 



Opponents of the toxin (or enzyme) theory of parasitism in the rusts 



