160 THE BOOK OF FORESTRY 
air. trees find it quite difficult. The fine dirt from the 
pavements tends to clog the “stomata,” the tiny breathing 
pores on the under side of the leaves, so that the leaves, 
the stomachs of the tree, find it hard to digest the food 
which is brought to them. The above explains why 
trees in the city often die from no particular disease 
or attack of any kind. In addition, by lowering their 
power of resistance these adverse conditions make trees 
much more subject to insect and fungus attacks. The 
forester must know the habits and resistance of his 
trees in the forest and in the same way the city forester 
or arborist, as he is sometimes called, must know which 
trees will survive in a given location. 
Duties of a City Forester—While the training re- 
ceived at the average forestry school is intended to 
fit men for tending timber forests, yet many such men 
have gone into city forestry work because they knew 
the life habits of trees. When the need of officials to 
look after trees was first felt by the cities, men were 
often chosen for political reasons but on account of 
the expert knowledge required it was soon found that 
technical training was absolutely necessary. On ac- 
count of the length of time it takes to grow a tree, 
mistakes are extremely costly. If the wrong kind of 
tree is planted or if not enough growing space is given 
when trees are set out along a sun-beaten street much 
valuable time is lost, hence a well trained city forester 
can save much time and money. At present many of 
the men in direct charge of shade tree and park work 
are possessed of both technical training and experience. 
Several institutions in the East are educating men along 
these lines and in the best of them a thorough course in 
landscape engineering is given, so that the entire prob- 
lem of construction and maintenance of a shade tree or 
