THE QUESTION OF ROOT EXCRETIONS 265 



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 open- 

 ings 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 influ- 

 ence upon plant growth. It was shown that roots of wheat are in- 

 different 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 " cowpea- 

 sick " 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- 



