198 SHADE-TREES IN TOWNS AND CITIES 



climatic conditions, the tree will become so weakened that 

 it will succumb to the attacks of parasites. 



By examining a tree carefully and noting its condition as 

 compared with other trees of the same species known to be 

 in a state of full vigor, one may determine whether the 

 specimen is in a good state of health or not. The points to 

 consider are generally the following : the growth in height 

 as shown by the vigor and length of the shoots ; the growth 

 of the trunk in diameter; the color and mass of the foliage; 

 the time of the unfolding and shedding of the leaves ; the 

 development of the crown, whether regular in all its parts ; 

 and the mode of shedding of the foliage, whether simulta- 

 neous in all parts of the tree. 



Soil Conditions. There is no doubt that in the case of 

 city trees, causes of decay and death can in the majority of 

 cases, be traced to poor conditions of the soil, such as lack of 

 food and water and root suffocation. 



A disease known as "stag-head" or "top dry" fre- 

 quently results from lack of proper food in the soil, and 

 manifests itself by the gradual death of the top of the tree. 

 Where trees grow year after year and there is no addition to 

 the available soil foods, and where the earth is dried out by 

 the sun and grass, starvation necessarily follows. The tree, 

 therefore, gradually stops growing, the branches slowly die, 

 and other diseases set in, until finally the last branch is dead. 



A constant supply of proper food is necessary to prevent 

 this disease. The ground underneath the tree should not be 

 sodded. It should be worked and top dressed at frequent 

 intervals to keep up the food-supply. 



Imperfect circulation of air in the soil has an important 

 bearing on the health of trees. City trees are likely to suffer 

 a lack of aeration of the soil, because of pavements and 



