INJURIOUS INSECTS, FUNGOUS DISEASES, ETC. 199 



walks. The use of grills and the keeping of the soil loose 

 will help the exchange of gases between the soil atmosphere 

 and the air. 



Root asphyxiation is especially liable to occur along 

 streets where the ground water is only a few feet from the 

 surface. During prolonged rainy weather the water rises, 

 making the soil wet up close to the surface. The pavement 

 adds here to the evil of poor under-drainage, preventing 

 evaporation and aeration. 



Young trees set too deep are often killed, and covering 

 the earth about trees with soil a foot or more deep usually 

 results in injury, if not death from asphyxiation. 



Ulcers. — ^This general term will be made to include all 

 internal injuries and diseases which manifest themselves 

 externally by the flow of sap over the bark. A disease 

 known as slime-flux is characterized by the appearance of 

 various colored slimy masses with a decidedly acid odor 

 which start at or near wounds. They make their appear- 

 ance usually in the spring when the sap, containing more or 

 less sugar, flows from the wounds. In the sap a number of 

 forms of algae, bacteria, and fungi begin to grow and form a 

 slimy, dripping mass over the bark. The cambium beneath 

 dies rapidly, and if the disease is not checked the destruction 

 may extend completely around a tree, resulting in the death 

 of branches and sometimes of the entire tree. 



An exudation of sap sometimes occurs through splits in 

 the bark, not near any wounds, and which from the outside 

 appears to be perfectly normal and healthy. The liquid is 

 usually mucilaginous and fermented, and as it flows over the 

 bark it discolors it gray, brown, or reddish. The species on 

 which such a condition is most frequently noticed are horse- 

 chestnuts, elms, maples, poplars, and oaks. 



