RESISTANCE OF PLANTS 383 



The interrelations between plants and the fungi that infect 

 their tissues and feed upon their sap are usually termed ''para- 

 sitism," the parasite causing more or less harm to the host 

 plant. 



Injury is caused by different vital activities of the parasite. 

 Often it is due to the mthdrawal of nutritive substances from 

 the host plant, leading to its exhaustion. However, such a direct 

 injury is usually quite insignificant; for the green plant forms 

 organic substance with ease and in abundance, while the total 

 size of the parasite may be very insignificant compared with 

 the host plant, and thus the amount of substances it consumes 

 cannot cause great depletion of assimilates. More serious are 

 the disturbances of the normal physiological functions caused 

 by the penetration of the parasite into the tissues of the host 

 plant. It has been observed frequently that under the influence 

 of fungi, the intensity of respiration increases considerably, 

 resulting in greater expenditure of organic substance. Some- 

 times this increase in respiration amounts to 15 to 30 per cent, 

 but at other times it reaches 200 per cent. Such a rapid expendi- 

 ture of substance inevitably leads to an appreciable exhaustion 

 of the host plant and to a decrease of the yield. 



Affecting the leaves of the plant and injuring in particular 

 the chlorophyll-bearing tissues, many of the parasitic fungi 

 considerably decrease the intensity of photosynthesis of the 

 leaves. This decrease often attains 25 to 50 per cent; and, in 

 some cases, when the parasite develops very abundantly, it may 

 be even 70 per cent and more (Kuprevitch, 1934). Such a 

 decrease in photosynthesis together with the increase in respira- 

 tory intensity considerably depresses the growth of the affected 

 plants and creates unfavorable conditions for the development 

 and filling of the grain. An epidemic of rust causes the same 

 effects as wind burn and windscorch; Diz., the grain is shriveled 

 and poorly filled. In some instances, however, when the infec- 

 tion is not great, a fungous infection may produce stimulation 

 that leads not to a decrease but to an increase of the intensity 

 of photosynthesis. In such cases, infection does not hinder the 

 plant, and the relation between the parasite and the host plant 

 approaches symbiosis. As has already been emphasized in the 

 discussion of the interrelations between the leguminous plants 

 and the nodule bacteria, the boundary between parasitism and 



