1024 
Journal of Agricultural Research 
Vol. XXIV, No. 12 
mhibitive substance of the cell sap. It seemed highly probable that this 
substance present in the living cells of the host was responsible, in part at 
least, for the checking of the advance of the fungus after it had penetrated 
the fleshy scales. It seems logical, therefore, to conclude that the 
restricted parasitism of the fungus is not due entirely to substances 
lacking in the fungus, but perhaps in some measure to the toxicity of the 
cell sap of the host. 
EFFECT OF THE VOEATIEE ONION OIE ON THE FUNGUS 
I 
Experiments were next undertaken to determine the effect of the vola¬ 
tile oil of the onion upon the germination and growth of the fungus. For 
the germination studies clean glass slides were laid in Petri dishes, two 
slides in each dish. The expressed juice of white onions, diluted to 
various degrees as in the above experiments, was used. Two drops of 
this medium containing the spores were placed on one slide in each Petri 
dish, while on the second slide in each dish were placed two drops of dis¬ 
tilled water containing spores in suspension. The data secured from this 
experiment are tabulated in Table II. 
Table II .—Effect of volatile oil of onion extract on germination of spores of Colleto- 
trichum circinans 
Petri 
Slide 
No. 
Germination. 
dish 
No. 
Medium. 
Dilution. 
Experi¬ 
ment X. 
Experi¬ 
ment 2 . 
f I 
Onion extract. 
jundiluted . 
Per cent. 
f 0 
Per cent, 
0 
I 
{ » 
Distilled water. 
i 0 
0 
f ^ 
Onion extract. 
>i to 10 . . . . 
( 0 
0 
2 
1 4 
Distilled water. 
l 7S± 
/ 7S± 
1 7S± 
/ 7S± 
1 7S± 
25± 
95 ± 
95 ± 
95 ± 
{ i 
Onion extract. 
>I to 100 . . . 
3 
Distilled water. 
{ 1 
Onion extract. 
[l to 1 , 000 . . 
4 
Distilled water. 
The unusual and striking feature of these experiments is that in the 
Petri dishes containing the undiluted extract, no germination whatever 
occurred, either in the extract or in the drops of distilled water. This 
could be attributed only to the fact that a volatile substance from the 
expressed onion juice had been absorbed by the distilled water and had 
prevented the spores from germinating. The toxicity of the onion oil 
was thus clearly demonstrated. The toxic effect of the onion oil upon 
the spores in distilled water decreased as the onion extract was diluted. 
These experiments were repeated many times with essentially the same 
results. 
The effect of the volatile oil was demonstrated in another way. A 
suspension of spores was made in two tubes of melted potato agar, 
which were then poured into sterile Petri dishes. After the agar had 
hardened the plates were inverted. In the center of the inside of the 
cover of one plate a small amount of onion extract was placed. The 
agar was thus exposed to the volatile oil, but was not in contact with the 
extract. The other dish served as a control (PI. 2, B). In it the spores 
