The effect of dust, smoke, and gas fumes upon vegetation is well known and yet no considerable amount of study has been given to this subject, largely because it has not been considered an economic question. Surely the people who are compelled to live in the congested districts of our large cities are as much entitled to shade and greenery as any one else, and there is no question but that the health-fulness of the congested districts is lowered by the absence of shade and grass. By the use of those plants which can survive drought, smoke, and abuse, some sort of trees or shrubbery may be had almost any-where, except perhaps in the immediate vicinity of a steel mill or similar factories, where not even grass will survive. The first trees one comes to on the edge of the treeless districts which surround large steel mills are usually ailanthus or willow. The ailanthus is also the tree which most often appears in the closely built up sections of large cities, often providing the only greenery to be seen in whole sections of a town. Ashes, locusts, European planes, European lindens, and horse-chestnuts also seem to have the ability to withstand the summer droughts and the suffocating soot that proves disastrous to so many city trees. No rough-leaved tree nor one which requires much water should be used as a street tree in a congested, sooty district, because it is doomed beforehand to a lingering death, if it survives at all. Pin oaks and willows are useful only when they are assured of a reasonable supply of water during summer droughts.
Among the shrubs such smooth-leaved, hardy sorts as the lilacs, privets, golden bells, buckthorns, and barberries seem to withstand the drawbacks of smoke, soot, and drought the best.
Most of the coniferous evergreens have a hard time even existing in any closely built up town. The Colorado blue spruce, silver fir, Scotch pine, and dwarf mountain pine have withstood the soot and gas better than any others, and some recent experiments with the Carolina hemlock seem to show that it, too, will survive in the heart of a city, providing it receives a certain amount of care. The common arborvitae has generally proved a failure. The exact reason why conifers are so unsuccessful has not so far appeared. Their short life seems to be due to the accumulation of soot which clogs the pores of the leaves and slowly suffocates them. They transpire so much water also during the hot summer droughts that they need an excessive amount of moisture, and they need numerous showers or washings from the hose also to keep them clean and cool. As most evergreens growing under city conditions do not get any care they rarely succeed, and when they do live, they lose their colour and are therefore not recommended.
As a rule, native collected plants seldom or never succeed when taken directly into the congested city districts, and only those plants among the deciduous shrubs and trees which are smooth-leaved are to be recommended for trial.