176 
Journal of Agricultural Research 
Vol. XXX, No. 2 
alter the host tissue in advance of the 
hyphae, and in that process part or all 
of the toxic substances may be changed 
or rendered inactive. These facts 
naturally limit the value of the experi¬ 
mental data in answering finally many 
of the questions which arise in connec¬ 
tion with parasitism. 
In order to reduce to a minimum the 
influence of outside factors upon the 
results the procedure was standard¬ 
ized as much as possible. This in¬ 
volved a study of the relation of tem¬ 
perature, age of spores, and age and 
condition of bulbs to the toxic effects of 
the volatile substances. Having worked 
out these relations primarily with a 
single fungus (Colletotrichum circinans) ) 
the comparative study of a number of 
fungi was begun. 
METHODS 
The volatile substances were obtained 
from onion juice which was secured by 
grating the succulent onion bulb tissue, 
placing it on a dry filter paper, and 
allowing it to filter without pressure. 
In certain cases, when a large amount 
of liquid was desired, the grated pulp 
was first placed in cheesecloth and the 
juice pressed out before filtering. No 
appreciable difference in the toxicity of 
extracts obtained by the two methods 
was noted, but to avoid any possible 
variation due to this cause, a single 
method was used throughout any given 
experiment. 
A spore suspension of the fungus in 
question was made in sterile nutrient 
liquid, either potato or onion decoction. 
Drops of the spore suspension were 
placed on a glass slide in a moist 
chamber. Another slide containing 
one or more drops, as needed, of the 
expressed onion juice, was placed in 
the same chamber. Control chambers 
without onion juice were always 
included in the tests. In all these 
experiments Petri dishes of uniform 
size, lined with moistened filter paper, 
were used as moist chambers and 
definite amounts of expressed onion 
juice were used as sources of the 
volatile substances. 
Observations were made after 6 to 
24 hours, depending upon the rate of 
germination and growth of the fungus 
in question. In order to stop growth 
throughout a series at a given time, 
chromacetic killing fluid was added to 
the drops at the end of the period of 
exposure and the counts and measure¬ 
ments were made as soon as possible. 
In measuring the effect of the toxic 
substances upon the fungi the per¬ 
centage of spores germinating and the 
rate of growth of the sporelings were 
recorded. The former was determined 
by counting 100 spores or more per 
slide. The rate of growth was esti¬ 
mated by measuring and averaging 
the length of the thalli of from 10 to 
40 sporelings on each slide. 
FACTORS AFFECTING THE TOXICITY 
OF VOLATILE SUBSTANCES FROM 
THE EXPRESSED JUICE 
METHOD OF EXTRACTION 
As previously stated, no appreciable 
difference was found in the extracts 
secured by the two methods used, 
namely, (1) by grating the succulent 
bulb tissue and allowing the liquid to 
filter without pressure, and (2) by 
extracting the juice from the grated 
tissue through cheesecloth and then 
filtering. 
AGE AND CONDITION OF BULBS 
Yellow Globe onions of a single lot 
were used in nearly all these experi¬ 
ments. They were secured in the 
autumn directly after harvest and 
stored in a basement the temperature 
of which remained at 12° C. or less 
during the winter months but gradu¬ 
ally approached room temperature 
after April 1. The investigations ex¬ 
tended from autumn until June. Dur¬ 
ing this period a reduction in the 
toxicity of onion extract was observed 
and this became especially noticeable 
after March. A similar reduction was 
noted in 1922 in onions of the previous 
autumn’s crop obtained in the mar¬ 
kets at Madison, Wis. It is to be 
expected that metabolic changes in 
the onion bulb continue during the 
protracted storage period. This appar¬ 
ently brings about some reduction in 
the volatile toxins, especially with rise 
in temperature. A marked difference 
in this respect was also noted between 
sprouted and unsprouted onions during 
the latter part of the storage period. 
Juice from sprouted onions has always 
been found to be less toxic than that of 
unsprouted onions from the same lot. 
AGE OF CULTURES 
Early experiments in which uniform 
results were not always secured when 
spores were not taken from a common 
source indicated that the age of the 
spore affected the degree of its resist¬ 
ance to the volatile toxins. In order 
to establish the importance of this 
factor, the spores of both Colleto - 
trichum circinans and Botrytis allii 
from potato agar cultures of different 
ages were compared under identical 
conditions. The data in Table I show 
