TREES IN TOWNS 4 
on 
Trees should never be planted in narrow streets, and 
can only safely be planted when the footpaths also are not 
narrow. It is objected to trees that they render the 
thoroughfare moist ; but such moisture as they occasionally 
cause does no harm to the surface of the roads, and on the 
contrary keeps them in better condition than dry dust 
would do. Some people do not like the fallen leaves of 
trees, but the trouble of removal is very slight. 
That towns are unhealthy to vegetation and especially 
to trees is well known; and this subject is relevant, as 
illustrating the strange fact when one thinks of it, that 
towns owing to various causes are unhealthy, just in the 
same way to human beings as they are to trees. First, 
there is the smoke of towns, which is a wicked waste of 
coal and entirely preventable. Evergreen trees in London 
are quickly coated over in winter with a dark shining 
carbonaceous product (2), soot, and need washing in the 
Botanic Garden of Regent’s Park. The smoke of towns (3) 
causes the pores of the leaves to be blocked, thus checking 
transpiration. Every dirty plant is practically living in 
twilight, as the soot reduces the assimilation of carbon 
dioxide by the leaves, depriving them of their most 
important source of food. Last of all, the sulphur contained 
in ordinary coal when burned is changed into sulphur 
dioxide, which ultimately forms sulphuric acid, a deadly 
corrosive poison to the leaves (4). 
The soil under streets in towns is unfertile, and is often 
very poor, being composed of building debris, ete.; but its 
main defect is its dryness, as owing to the modern 
pavement all the water, which falls as rain, runs off into 
the sewers, finding its way into the sea, and never reaches 
the soil, where the roots of the trees are. Drought is the 
greatest enemy of trees, as the latter, owing to their 
extensive surface of foliage, require enormous quantities of 
water. The only trees that thrive in streets in towns are 
those able to resist drought. 
The soil, according to Wieler, being made acid by the 
action of the sulphur compounds in the soot, loses its 
