46 FORESTS, WOODS, AND TREES 
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 
