DISEASES OF SHADE AND FOREST TREES. 119 



treatment must be used with care, as it will sometimes re- 

 el iu-c the germinating power of the seeds. 



Insects are the cause of many diseases, among the most 

 interesting of which are the cecidia or galls. These ab- 

 normal growths occur on roots, stems, leaves, flowers and 

 fruits and are due to insect injuries, in most cases the insects 

 making their homes for a considerable part of their lives 

 within the iralls. Some of them fire very in- 



Galla not 



iurious, but most of those occurring on trees often 



harmful. 



are considered ol little importance. However, 

 individual plants are frequently so seriously affected as to 

 mar their beauty and no doubt reduce their vitality. The 

 great regularity of shape, color and markings of these galls 

 will always make them objects of great interest to both 

 scientist and layman, and future studies will probably prove 

 them to be the cause of greater injuries than we now at- 

 tribute to them. Fortunately, owing to the migratory char- 

 acter of the insects, most species of insect galls do not occur 

 two or more years in succession on the same trees. Some 

 few species can be sprayed to advantage with insecticides, 

 but in most cases where the pests become troublesome it will 

 be found necessary to prune and destroy the diseased parts 

 before the insects emerge. 



Abnormal structures are also formed as a result of fungus, 

 bacterial and mechanical injuries. 



Animal injuries of various kinds are frequently the open- 

 ings by which fungi and other organisms of disease gain 

 entrance to the host plants. Insects, birds, squirrels and 

 other animals are also the distributers of many 

 diseases bv making wounds and carrving the promote 



,, disease. 



organisms from place to place. Storms also 

 aid in the work of destruction by breaking branches and thus 

 causing wounds which immediately become sources of infec- 

 tion. 



