entered their resting period, the growth of wheat in the pots became better. It does not seem possible that the harmful effects of the trees could have been due to the removal of plant food; if so, there would have been no increase in the yield in autumn. It seems, therefore, that the presence of the roots must have had some other effect upon the growth of the wheat, as the size of the pots made it necessary for the two kinds of roots to be in close physical relation. That the retarding effect is due to substances excreted by the tree roots seems probable. It was also noted that tree pots that produced as much wheat growth in November as the controls were the ones in which the trees showed the earliest signs of winter rest. These results seem to be conclusive, therefore, in showing that the injurious effect of the trees was due to the excretion of toxic substances from the roots.
These experiments, which may be regarded as furnishing negative evidence upon the problem, were supplemented by others which furnish more direct and conclusive evidence of a positive sort. By taking advantage of the chemotropic sensitiveness of the roots of seedlings, it is possible to employ them as indicators of the presence of their own harmful excretions. It is well known that roots, like other perceptive organs, will curve and grow towards substances possessing beneficial properties, but will curve away from other bodies possessing deleterious properties. Advantage was taken of this reaction to chemical stimuli for showing the presence of deleterious excretions.
Without going into lengthy details of the experiments in this place, it may be said that the method consisted essentially in growing roots in small glass tubes, from which they might escape at suitable openings if any stimulus caused them to turn aside from their normal downward course of growth. In this case there was a stimulus, and it was the deleterious matter excreted from the roots during growth. It was unmistakably shown that the growing roots of healthy plants do excrete deleterious substances in amounts sufficient to exercise an influence upon plant growth. It was shown that roots of wheat are indifferent to substances excreted by plants of another sort, like maize or cowpeas. The excretions from roots of a more closely related plant, like oats, were not as harmful to wheat as excretions from wheat, but more harmful than excretions from the more distantly related plants.
Eecent work in the laboratories of the Bureau of Soils has resulted in obtaining organic compounds from "wheat-sick" and "cowpeasick" soils. At the late meeting of the American Association for the Advancement of Science, held in Chicago, Schreiner and Sullivan reported the results of this work.
Wheat seedlings were grown continuously upon a soil until it became "wheat-sick," and its productiveness for wheat was quite small. This soil when carefully distilled yielded a small quantity of a crystal-
