46 FOKESTS, WOODS, AND TKEES 



bacterial activity, and becomes unfertile. The addition of 

 new soil is often favourable, as it inoculates the old soil. 

 Chemical works in towns, which pour into the atmosphere 

 fumes of hydrochloric, nitric, and sulphuric acids, are fatal 

 to all trees within a certain radius. The new methods of 

 oiling and tarring roads are probably injurious to trees ; 

 but this fact is not yet quite established (5). Trees are 

 also killed in towns by the escape of ordinary lighting gas, 

 if their roots are exposed to the leak for some time. Trees 

 are also injured by the escape of electric currents. All 

 these make a long list of evil conditions to which trees are 

 subject in cities ; and there is little doubt that fine trees 

 in the centre of modern towns are decaying rapidly and 

 disappearing. I still think that the great enemy is drought, 

 want of water in the soil. To all these evils must be 

 added the injury often done to trees when the level of a 

 street is altered, when pipes are laid or changed, etc. In 

 fact trees in towns are in constant danger from ignorant 

 workmen. 



In the central or business quarter of a city or town, 

 where the ground is covered with buildings and pavements 

 involving the worst conditions of soil, the species that have 

 proved successful in the streets are very few in number. 

 Thus, the list of desirable street trees in Paris comprises 

 only eleven species, and in Washington twelve species. 

 In England the choice is even more restricted. Prof. Farmer 

 could recommend to the Kensington Town Council only 

 four trees, namely, plane, Ailanthus, Jersey elm, and 

 Lombardy poplar, though he suggested the trial of four 

 others, namely, common lime, single-leaf ash, red-flowering 

 horse-chestnut, and Bolle's poplar. 



In the residential area or suburbs, where gardens or small 

 plots often intervene between the houses and the pavement, 

 the roots of the street trees find copious supplies of air and 

 water at no great distance. The soil conditions are much 

 more favourable than in the centre of the town, and the list 

 of species that can be successfully planted is considerably 

 augmented. In parks and open spaces, where the soil is in 



