SCHREINER AND REED: EXCRETIONS BY ROOTS 301 
In view of the very potent effect of the excretions of the plants 
which have been studied it can only be concluded that the excre- 
tions from plants and the accumulation of such excretions in the 
soil are of the utmost importance in determining such phenomena 
as association, invasion, and succession. 
SUMMARY 
1. It has long been known that certain of the lower plants 
produce substances of an excretory nature which render their en- 
vironment unsuitable for further growth; but it is only recently 
that data have been presented to show that the roots of the higher 
plants may excrete substances which are deleterious to their further 
growth, 
2. The experiments related in this paper show that healthy 
growing plants excrete from their roots substances which have a 
deleterious effect upon the growth of the root. 
3. The excreta produced by the roots are so small in amount 
that, up to the present time, they have not been detected by chem; 
ical analysis. The chemotropic sensitiveness of the plant does, 
however, afford a means of detecting and demonstrating experi- 
mentally the presence of root excreta. 
4. The experiments described in this paper show that, as a 
tule, the excreta produced by a plant are most toxic to plants of 
that same species. So far as studied the excreta are more toxic 
to closely related species than to distantly related ones. Obser- 
vations in the field indicate that there are specific instances in 
which the excreta of one species are extremely toxic to other dis- 
tantly related species. 
5. The production of toxic excretions by the roots of the higher 
plants appears to afford an explanation of some of the important 
phenomena connected with association, invasion, and succession of 
plants. It is no less important as an explanation of certain un- 
derlying principles in agriculture, chief among which are those 
of crop rotation and the productivity of the soil. 
Bureau oF Sorts, U. S. DEPARTMENT 
OF AGRICULTURE, WASHINGTON, D, C. 
