SHADE TREES. 125 



of destructive insects and fungous pests, which heretofore have not been 

 troublesome, has rendered the problem still more perplexing. 



Some of the factors which enter into the problem of selecting species 

 and varieties for shade trees are the following: 



Adaptability to Climatic Conditions. 



One of the first requisites in selecting a tree for street planting is a 

 knowledge of its climatic requirements. Many species of trees are likely to 

 suffer from extreme meteorological conditions, and even species indigenous 

 to a certain region may prove a failure when planted in a city or town as 

 shade or street trees. There are also certain species which have their 

 limitations as regards climate, such as some Japanese varieties, and in 

 planting this should be taken into account. Under adaptability to climatic 

 conditions is included the ability of a species to withstand the detrimental 

 effects resulting from heat and cold, wind, snow and ice. atmosphere and 

 soil moisture and light intensity. 



Hardiness and Resistance. 



Hardiness and resistance are the capacity of a species to withstand 

 extremes of climate and the more or less abnormal and severe conditions 

 of the particular environment in which it may be placed. These may 

 arise in part from the peculiar atmospheric and soil conditions which 

 are characteristic of congested settlements where the soil has been made 

 from various types of refuse, or may be due to the presence of large man- 

 ufacturing establishments. 



Configuration and Conformity. 



The shape or form of a species, as well as its conformity to its environ- 

 ment, is essential. Wide avenues demand different species from narrow 

 avenues; ajid the habit of branching, root development, height, spread 

 of the crown and general symmetry of the tree should be considered. 



Longei itij. 



The age which a species is capable of attaining is important in its 

 selection for planting, and while short-lived trees may have their use in 

 certain places for temporary growth, a longer-lived variety should be 

 selected for permanent effects. While the causes underlying senescence 

 and rejuvenescence are hereditary in individuals, the life and usefulness 

 of a tree may be prolonged by treatment, and its configuration greatly 

 modified. Some trees, such as the apple, are readily rejuvenated, while 

 others respond very poorly to treatment. 



Rapidity of Growth. 



The growth of trees in general is quite variable. Even individuals of 

 the same species are different in this respect. Much also depends on 

 environment in the growth of trees. The modern tendency in tree plant- 



