290 SCHREINER AND REED: EXCRETIONS BY ROOTS 
Controls were set up simultaneously which contained freshly pre- 
pared agar. Each lot of jars received a set of segmented glass 
tubes and when the agar had cooled to the proper temperature, 
wheat seedlings were inserted in the open ends of the segmented 
tubes. If the foregoing hypothesis regarding the presence and 
diffusion of toxic waste products be true, one would expect fewer 
roots to leave the segmented tubes when they were surrounded by 
a medium which already contained the waste products, because 
there would be a more equal concentration of them throughout 
the entire medium. This is precisely what was observed. Of 23 
roots which grew in the used agar, only 8 curved and grew out of 
the tubes. Of 13 control roots, 7 turned and grew out of the 
tubes. 
Do the roots in the tubes curve because of an insufficient supply 
of oxygen? There might be some doubt as to whether part of the 
response might arise from a deficient supply of oxygen in the small 
tubes, and the question ‘“ Do the roots in the tubes curve because 
of a possibly insufficient supply of oxygen?” naturally presents 
itself. The evidence obtained by Bennett (’04) is entirely against 
such behavior. The author just mentioned made very careful and 
exhaustive experiments with the roots of land plants, but was 
unable to find any evidence whatever that direction curvatures 
could be induced by the one-sided application of such gases as 
oxygen or carbon dioxide. It was deemed desirable, however, 
to test the roots of wheat seedlings in the experiments under con- 
sideration. A number of segmented glass tubes were set up in 
jars of freshly prepared agar and a wheat seedling placed in the 
upper end of each tube. When the agar had become partially 
solidified, air bubbles were produced by manipulating a glass tube 
in it, The bubbles were formed at the surface of the segmented . 4 
tubes and at the level of the narrow openings. When the roots 
grew downward, they showed the usual curvature at the openings. 
They did not, however, show any tendency to grow toward the 
air bubbles, nor to the agar in the vicinity of the air bubbles. 
Many roots grew directly through air bubbles and passed on with- 
out being deflected from their course by the presence of air bubbles. 
It would be manifestly wrong to base judgment on this question 
if the roots passed from the agar in the air bubble and remained 
