34 
TENTH REPORT. 
one hour to another. But it is highly probable that the character of these 
complex substances set free by roots, is peculiar to the species of plant. 
These substances, including the liquid excreted by roots, the decomposing 
loose cells and the bacteria and fungi, seem to act injuriously towards 
the roots of other plants when such roots are immersed in liquid con- 
taining them. And the experiments showed that material excreted from 
corn checked the growth of the squash roots more than material excreted 
from the roots of the squash checked the growth of the squash. 
Working on the assumption that such excretory substance was the cause 
of the phenomena mentioned, it was concluded that it would be a compara- 
tively easy matter to remedy the culture medium (distilled water). Several 
means were employed (1) close filtering; (2) shaking up with filter paper, 
paraffin pellets, gypsum; (3) passing oxygen through the solution; (4) adding 
a little hydrogen dioxide. Carbon black was not employed, because it had 
been used with great success at the Department of Agriculture, Washington, 
and elsewhere, and no doubt it would be as effective as any here mentioned. 
Moreover, the writer has used carbon black off and on for many years for 
absorption experiments, and for such work as that now employed, so it 
was thought unnecessary to apply it here. Pieces of coke are very conve- 
nient and useful, especially for suspended material and loose cells. 
The action of all the.se substances is partly a physical one. The substances 
in the liquid, whether in solution or not, are attracted to the filter paper or 
gypsum and held there mechanically. The root liquid can then be improved. 
But the most important effect of these inert bodies on the liquid is to remove 
the excreted loose cells, and not so much on the substance in solution. This 
is borne out by the fact that a liquid, shaken up with gypsum, talc, etc., 
contains still small quantities of substance in solution, but this substance 
does not seem to be injurious to the roots, at least not for a considerable 
length of time. 
The most remarkable operation is that of the application of oxygen, 
whether used directly or in the form of hydrogen dioxide. It seems that 
its action is to oxidize bacteria and also the cell content of the loose cells, 
and thus produce compounds not directly injurious to the roots of the plant 
excreting them. It is now known that plant roots are themselves oxidizing 
agents, and this fact throws light on the whole question. When oxygen 
is passed into two culture liquids which have been in use for a week or ten 
days, the one used with corn and the other with squash, it improves both, 
but it improves the corn root liquid more for corn than it does for squash, 
and the squash root liquid more for squash than for corn. In other words, 
it appears as though the oxidizing action of a plant is more effective against 
excreta of its own roots, than against those of another species. The pro- 
ducts of oxidation are apparently then not the same for the two plants. 
One might suggest that this is a result of adaptation, but perhaps this might 
seem to be too “ far-fetched . ” 
To account for the augmented growth during the first period, is not such 
an easy matter, but it might be explained on this basis. During the first 
twelve hours, there could be little decomposition either bacterial or from 
direct oxidation. The liquid excretion from living cells of the radicle is 
apparently the only substance to be taken into consideration, and it seems 
probable that this excretion might be stimulating to the growth of the radicle 
of another species which might be present, and yet more or less inhibitive 
to the radicle itself. This is more or less theory, because it is very difficult 
to get experimental proof, but it should be remembered that the only point 
