436 



ELEMENTARY STUDIES IN BOTANY 



tree appears as though its branches had been badly scorched 

 with fire. This disease is caused by certain bacteria that 

 spread through the living cells' and destroy them. It is 

 always necessary to determine how a parasite effects an 

 entrance in a plant, for this suggests the method of preven- 

 tion. The entrance of a parasite is spoken of as an " infec- 



FIG. 85. A spot disease of currant leaf. After HALL. 



tion," and in the case of pear blight, it is found that the 

 infection is brought about by insects (especially bees) visit- 

 ing the flowers. This infection spreads to other flowers and 

 through them into the young twigs. Just how the insects 

 get the bacteria is a detail that is not needed for practical 

 purposes. 



Spot diseases. It is very common to see leaves spotted, 



