SCHREINER AND REED: ExcRETIONS BY ROOTS 293 
glass tubes. Evidence is displayed to show that the curvature 
of the roots is not due to the action of light, of gravity, of water, 
nor to a lack of oxygen. Since the medium employed as a soil is 
as nearly as possible non-nutritive, the question of plant nutrients 
does not complicate the study. When the effect of gravity is 
neutralized, a larger proportion of the roots respond to the stimu- 
ulus produced by the presence of toxic excreta. 
EXPERIMENTS SHOWING THE BEHAVIOR OF WHEAT PLANTS .GROW- 
ING IN A MEDIUM CONTAINING THE EXCRETA FROM THE 
ROOTS OF WHEAT OR OTHER PLANTS 
With the foregoing results in hand, the next problem was to 
Study the behavior of one plant in the presence of excreta from 
various other plants. Wheat was selected as the plant to be em- 
ployed as an indicator, since the roots which it puts out after the 
short life of the primary root are positively geotropic and suffi- 
ciently sensitive for chemotropic reactions. 
It was shown in the preliminary experiments that the toxic 
effects remained in the agar in which the plants had grown. It 
was also shown that the toxic effects persisted when such agar was 
melted and used again. In the experiments which are next to be 
described the effects of root excretions from wheat, corn, cowpeas, 
and oats were studied. The agar containing their excretions was 
obtained in each case by planting a large number of seedlings in 
a dish of soft agar and allowing them to grow for 8 to 15 days, 
according to the plant employed. Ina few days the agar was com- 
pletely permeated by the roots of the plants, the plants being 
nourished in the meantime by the reserve materials of the seed. 
When the agar was to be used for experiment the plants were 
pulled out, removing as little agar as possible. The agar was then 
placed in a shallow, covered dish on the steam-bath, stirred and 
heated to 75~80° C. Continued heating at a high temperature 
changes or destroys the toxic substance ; even at this temperature 
_ Some of the toxic effect was probably lost. As soon as the agar 
Was completely melted it was filtered through absorbent cotton into 
the desired receptacles and cooled to room temperature. Distilled 
Water was added to restore that lost by evaporation. 
The following series of experiments were so designed that the 
