184 
Joumal of Agricultural Research voi. xxx, no. 2 
In comparing first the reaction of the 
several onion parasites (fig. 3), it is to 
be noted that there is again a rough 
negative correlation between aggres¬ 
siveness of parasitism and sensitive¬ 
ness to the volatile toxins. As evi¬ 
denced both in germination and growth, 
Colletotrichum circinans is the most 
sensitive; Aspergillus niger is next; 
both of these are very weak parasites. 
The reactions of the nonparasitic 
species do not lend themselves to any 
very general statement. Lack of path¬ 
ogenic properties upon the onion is not 
necessarily correlated with high sensi¬ 
tiveness to volatile oils. In this case 
at least Botrytis drierea was stimulated 
in germination and growth by exposure 
to the volatile oil. Fusarium grami- 
nearum, Colletotrichum lindemuthia - 
Fig. 3.—Graph representing the growth of sporelings of several onion-bulb parasites, when exposed to vari¬ 
ous concentrations of volatile onion oil. See data in Table VII. Note that the two weakest parasites, 
Colletotrichum circinans and Aspergillus niger are the most retarded by the volatile toxins as compared 
with the other four forms which are aggressive parasites of the onion bulb 
In contrast to this, the more aggressive 
bulb-rotters ( Botrytis allii , Fusarium 
cepae, Fusarium sp. 45, and Asper- 
gillus sp. 4660) all show more or 
less germination and growth in the 
highest concentration of toxin. The 
trend therefore coincides in a gen¬ 
eral way with that noted for dissolved 
toxins (Table VI). Botrytis 108a oc¬ 
cupies an intermediate position be¬ 
tween the two groups, being a rather 
weak parasite, but still exhibiting con¬ 
siderable resistance to the volatile 
toxins. 
num f and C. trifolii were distinctly 
inhibited, while C. pisi and Glomerella 
cingulata were only slightly retarded. 
DISCUSSION OF RESULTS 
In the establishment of the parasitic 
relation the factor most important to 
the invader is the presence of food, and 
one of the principal opposing factors is 
the presence of toxic substances within 
the host tissues. Since different spe¬ 
cies of fungi vary in their response to 
these antithetic factors, the question 
