Minnesota Plant Diseases. 89 



Host-influence on parasite may be exerted even at the time 

 of spore germination. The spores of a great many parasitic 

 fungi will start to grow when placed in pure water. Some 

 fungi, however, as most of the smuts, require nutrient material 

 before they will germinate. Again, other fungi must bring 

 their spores into direct contact with the host plant in order to 

 bring about germination. 



After the spore has germinated the germ-tube penetrates 

 the tissues of the host plant. In some cases, where the fungus 

 spore is not directly influenced in its germination by the pres- 

 ence of the host, a spore may germinate on an unfavorable 

 host and the germ-tube may even penetrate into the tissues, 

 but here its progress is prevented as the host does not permit 

 of further growth. Such a struggle may continue for some 

 time, but is usually short if the host is at all unlike the usual 

 host of the fungus. The preference shown by fungi for special 

 plant parts as dwelling places, as the grass-fruit-inhabiting 

 smuts, is but another expression of the influence of the host 

 plant parts upon the fungus. 



