nize and remove the cause of disease. Between gas in the soil and gases in the air urban trees lead a more or less poisoned and morbid existence, appreciated and enjoyed only by those who can not or will not go where trees grow rapidly and well, solitary or gregarious, according to their kinds, a delight to the eye, an inspiration to the spirit, a shelter to the birds, and a satisfaction to the lumberman.
Perhaps this seems an exaggerated statement of urban conditions, but an examination of the trees in the parks of Pittsburgh, Cincinnati, St. Louis and Chicago, among the industrial cities using mainly soft coal, and of New York and Boston, in which hard coal is the principal fuel, will convince the fair-minded observer that something is wrong.
Air and soil analyses and experiments under controllable conditions, will alone reveal the cause of the condition of urban trees and of those growing near industrial plants from which poisonous fumes emanate. Such analyses of air and soil can be made only by a skilled chemist, and such experiments should be attempted only when a skilled chemist and a trained biologist can cooperate under such conditions of climate and equipment as to justify the expenditure of time and money.
In the remarkably pure air of Stanford University, with the nearest domestic coal fire a quarter mile away and with gas supplied to the university by a single small main, a greenhouse has been so constructed that it is completely divided into two exactly similar halves, in which experiments and their controls can be carried on under as nearly natural conditions as can be conceived. In each half of this house there were boxes of growing grain, a considerable variety of potted seedling trees, one, two and three years old, and other plants, aquatic and terrestrial. The grain was grown on the spot from seeds; the trees were grown from the seed in the pots and were brought into the house so long before they were used in experiments, that they were quite accustomed to their surroundings. Into one half of the experiment house, a dose of sulphur dioxide equal to the proportion in pure city air, was introduced daily (Sundays excepted) for months, during the resting period only for one set of trees, during the growing period only of another set, and during the resting and most of the growing period of a third set.
The trees continuously in the side of the house dosed daily with a very minute quantity of sulphur dioxide gas, blown into and thoroughly mixed with the air of the house, showed a growth in length two thirds of that of the trees in the pure air of the other half of the house. There are no visible injuries; only measurement shows that those trees in air purer than that of most cities do not thrive, do not make as much growth as trees in thoroughly pure air. Analyses show that the sulphur dioxide is fixed, mainly in the leaves, and thus disturbs the vital activities of plants. The results with grain correspond. These results obtained by experiment justify, therefore, the assertion that the problem
