SCHREINER AND REED: EXCRETIONS BY ROOTS 299 
roots which left the tubes in experiments ‘“‘a” and “6” was 
80 :100, showing that there was a sensible difference whether the 
used agar was within or without the tubes. From these relations 
we may conclude that the excreta of oats are more toxic to the 
roots of wheat seedlings than those of corn or cowpeas, a con- 
clusion which is substantiated by the results obtained in crop ro- 
tation. If we represent the responses of the roots in the different 
experiments in the form of percentages, we obtain the following: 
Wheat succeeding wheat, 44 per cent. 
Wheat succeeding oats, 80 per cent. 
Wheat succeeding corn, 95 per cent. 
Wheat succeeding cowpeas, go per cent. 
Summing up the results of these experiments which demon- 
strate the effect of root excreta from various sources upon wheat 
roots, it may be noted (1) that the excretions from no other roots 
were so deleterious to wheat as its own excretions, (2) that the 
excreta from oats were more harmful than those from the more 
distantly related plants, cowpeas ‘and corn, (3) that the plants 
which succeed best in a rotation of crops with wheat, produce 
excreta which are least harmful to wheat. 
THE ROLE OF TOXIC EXCRETA IN THE ASSOCIATION AND 
SUCCESSION OF PLANTS 
The production of such toxic excreta as have been demon- 
strated in this paper throws light upon the problem of association 
and migration of species and individuals in the vegetable kingdom. 
The problems of natural association and migration among plants 
were, as one of us has shown in another publication (Reed '05), 
among the first studies of ecologists. It has long been known 
that various physical ‘factors, ¢. g. light, water, etc., often determine 
the limit of the range of a given species, but it has been repeatedly 
admitted that they are not sufficient to explain certain important 
problems of association and succession. The importance and 
activity of biological factors cannot now be overlooked by any 
student of ecology. The working of root excreta in causing as- 
sociation and succession are admirably illustrated by the investi- 
gations of the Woburn Experiment Station, Jones and Morse, and 
Jensen, cited in the first part of this paper. 
