434 



ELEMENTARY STUDIES IN BOTANY 



killed. A plant attacked by such a parasite may live along 

 in a more or less enfeebled way, or it may be destroyed 

 completely. 



In a second group, the parasite does not kill living cells, 

 but lives in association with them, feeding upon their prod- 



FIG. 83. A spot disease of apple leaf. After SORAUER. 



ucts. Often as a result of the presence of such a parasite, 

 the living cells are " stimulated " into doing unusual things, 

 such as the development of " galls " or other unusual growths. 

 Such growths are symptoms of the presence of such a para- 

 site. This peaceful living together is usually brought to an 

 end when the parasite begins to form spores. 



