Brown. — St tidies in the Physiology of Parasitism. IX. 299 
The question of a specific action on a particular fungus of the volatile 
substances from a particular plant has scarcely been touched. This is 
a problem of considerable experimental difficulty, and as such it has been 
decided not to follow it up for the present. The difficulty in question is 
one of special methods rendered necessary by the nature of the problem. 
The active substances are volatile and produce their effects when present in 
small quantity. A quantitative examination would therefore involve con- 
siderable technical difficulties. As a further difficulty one would have to 
reckon with the strong probability that the curve expressing stimulation 
in terms of concentration would show a pronounced optimum, so that in 
practice one would not be certain, short of considerable experimentation, 
whether a small effect in any particular case was due to too much or too 
little of the stimulant. The interpretation of the result in any particular 
experiment would thus not be easy, especially as at the present there seems 
to be no ready means of regulating or measuring the concentration of the 
stimulant. As an illustration of this difficulty we may refer to the experi- 
ment quoted on p. 294. The presence of apple tissue was there seen to 
produce distinct stimulation of Botrytis cinerea spores, but to inhibit spores 
of Colletotrichnm. There would appear to be a certain appositeness in this 
result — that Botrytis spores are stimulated by volatile substances arising 
from apple fruit, which is one of its natural hosts, whereas Colletotrichum , 
which does not occur on apple, is not so stimulated. But this conclusion 
would be premature, as it is quite possible that under suitable conditions 
the spores of the latter fungus could also be stimulated by the presence of 
apple tissue. Obviously a considerable amount of work is still required in 
this connexion. 
Summary. 
1. The germination of Botrytis cinerea spores is increased by the action 
of volatile substances arising from certain plant tissues, such as apple leaves 
and fruit, leaves of Rut a , Eucalyptus , &c. 
2. In other cases reduced germination or even inhibition is produced — 
viz. with tissue of potato tuber, onion leaves or bulb scales. 
3. Reduced spore germination due to volatile substances produced by 
growth of organisms in wet filter- or blotting-paper is described. 
4. Similar stimulating and retarding effects can be produced by the 
action of simple chemical substances such as ethyl acetate. 
5. A number of other fungi were tested in these respects and found to 
behave similarly to Botrytis cinerea. 
6 . The bearing of these results on general mycological technique and on 
problems of physiological parasitism is discussed. 
