286 SCHREINER AND REED: EXCRETIONS BY ROOTS 
further growth and that its inhibitory effect was first felt by the 
delicate germ-tubes. 
Wehmer (’g1, ’06) has demonstrated that certain of the fungi do 
produce a very toxic substance, namely, oxalic acid. 
PLAN OF THE EXPERIMENTS 
One general feature of the foregoing work upon excretions 
has been the indication that the amount of substance excreted is 
very small. The removal of toxic substances by the use of a 
small amount of absorbing material or by momentary boiling are 
evidence that refined methods are necessary in their investigation. 
The amount of toxic substances present is so small that an ordi- 
nary chemical analysis of the soil does not give evidence of them. 
The assumption on the part of Braconnot (’39), Walser (’38), 
Boussingault (’41), and others that these substances existed in 
sufficient amounts to be detected by ordinary analysis was a funda- 
mental error, which kept other workers from investigating their 
nature and action. At the same time there is evidence which can- 
not be overlooked that the plant is sensitive and does respond to 
the presence of deleterious substances. The following experi- 
ments were therefore so planned that the plant itself could be used 
as an indicator of the excretion of deleterious substances from its 
roots. 
For indicating the presence of a small amount of deleterious 
material, the rate at which the root of a plant elongates is not en- 
tirely satisfactory. Experiments like those of Lilienfeldt (’05) and 
especially those of Fulton (’06) show that the chemotropic behavior 
of an organism can be depended upon to indicate the presence of 
small amounts of deleterious substances. Seedlings of wheat and 
oats (Zriticum vulgare and Avena sativa) were used as indicators 
in most of the experiments, since it was found that their roots were 
chemotropically sensitive to the deleterious substances studied. 
The wheat seedlings used were germinated in water by a method 
devised in the laboratory of the Bureau of Soils, and described by 
Livingston (’06). The seedlings germinated by this method were 
remarkably uniform, and their roots were straight and free from 
any adhering particles. The design of the experiments included 
a study of the behavior of the roots of wheat seedlings in the 
