SCHREINER AND REED: EXCRETIONS BY ROOTS 291 
there, since the environment would be changed in respects other 
‘than air content. One might expect, however, that if an aerotropic 
tendency were manifested it might be shown by roots turning 
toward the agar in the vicinity of the air bubbles. Such a ten- 
dency was not manifested by roots in this or other experiments 
where air bubbles were present, hence it follows, that the results 
reported are really due to a stimulation of the roots by some dele- 
terious substance and not to a deficiency of oxygen. 
The behavior of the roots of oat seedlings in the presence of their 
own excreta. — For comparison with the behavior of the roots of 
wheat seedlings a series of tubes was prepared which held oat 
plants. The segmented tubes contained and were surrounded by 
freshly prepared agar. The experiment lasted 6 days. The agar 
in the tubes became so toxic that 13 of the 39 roots employed 
were killed. Of the remaining roots, 19 curved and grew out of 
the tubes. This is a response of 73 per cent. (the roots killed 
_ Not being considered), and shows that the roots of oat seedlings 
produced a toxic condition which was quite repellent to them- 
selves. In all cases it was the later roots which were killed. 
The roots which grew out into the surrounding fresh agar showed 
no signs of injury. 
Experiments in which malic acid was added to the culture med- 
ium, — Continuous observation of the agar in which plants were 
grown spoke against the action of the bacteria as a partial factor 
in producing the results. Nevertheless it was thought profitable 
to make an experiment in which bacteria were excluded. 
After some preliminary experiments which showed that wheat 
seedlings could tolerate malic acid, a quantity of wheat seedlings 
Were grown in agar which contained 125 parts per million of malic 
acid. While this amount of acid was not sufficient to inhibit the 
. growth of wheat roots, it reddened litmus paper instantly, and it 
is safe to assume that bacteria did not develop in such prepara- 
tions. Part of the segmented tubes contained this agar in which 
plants had been grown and the others were surrounded by it. 
Fifteen roots grew in segmented tubes containing fresh agar 
and surrounded by used agar. Of this number only a roots 
Stew out into the used agar. Thirty-three roots grew in seg- 
Mented tubes containing used agar and surrounded by freshly 
“ 
