: 
SCHREINER AND REED: EXCRETIONS BY ROOTS 295 
been grown and surrounded by fresh agar. The results of six ex- 
periments are presented showing that 62 per cent. of the roots 
capable of responding grew out of the tubes containing the agar 
in which a previous wheat crop had grown into the fresh agar 
surrounding the segmented tube. 
Experiment No. pag oe ne of se ig oc Per Cent. Response. 
I 8 7 87 
2 13 4 3f 
3 II 6 55 
4 8 27g 75 
5 | gaint 16 48 
6* 41 30 73 
Average response for the six samples of agar, 62 per cent. 
It will be noted that the percentage of diverted roots in the 
several experiments varies somewhat. This is due to the varying 
toxicity of the agar derived at different times and from different 
crops, as shown by the fact that the differences are consistent in 
the two series, namely, that a high result in series “a” is always 
accompanied by a low result in series “4” and vice versa for the 
same sample of used agar. 
In the development of roots and tops the plants in series 6” 
excelled. This is probably due to the greater total quantity of 
fresh agar in that set. As soon as the roots passed out of the 
segmented tubes the sensitive and absorbing portions were in a 
medium containing a relatively small amount of toxic excreta. It 
is obvious from these data that toxic substances remain in the used 
agar and exert an influence on the succeeding crop. When the 
roots growing in freshly prepared agar reached the openings in the 
segmented tubes 27 per cent. of them grew out into the used agar, 
Whereas under the opposite set of conditions 62 per cent. of the 
Toots grew out of the tubes, the proportion of the responses 
being 44: 100. This plainly indicates that in the agar which had 
§rown a crop there is some toxic substance, the only source of 
Which could be the excreta from the roots of the previous crop. 
WHEAT SUCCEEDING CORN. 
The corn used in these experiments was a variety of pop-corn, 
a 
. a ‘ . oe] . ies, 
* Experiment 6 has no corresponding experiment in table ‘‘a’’ of this series 
