298 Brown . — Studies in the Physiology of Parasitism. IX. 
take steps to reduce disturbing atmospheric factors to a minimum. In 
cases where relative effects only were being considered, any disturbance 
arising from volatile substances would probably be of less consequence, 
but if it was sought to determine the absolute amount of germination, the 
result obtained, e. g. in the presence of a large number of other cultures, 
would in all probability not represent the intrinsic germinating capacity of 
the fungus under investigation. In particular the custom of placing moist 
paper in the lids of Petri dishes in accurate studies of germination is not 
to be recommended unless proper control measures are taken. Further- 
more, one may ask in this connexion how far the failure, so often reported, 
to obtain vigorous attack under laboratory conditions with organisms which 
readily produce attack under field conditions may not be due to some such 
influences as are here indicated. 
The possible significance of the results of the present paper for the 
physiological analysis of pathological problems will now be discussed : 
(r) in relation to the conditions in the infection drop, (2) in relation to 
growth of the fungus after penetration of the host. 
It was shown in earlier papers of this series that the conditions present 
in the infection drop are in general those of feeble nutrition. The vigour 
of germination in the infection drop — a factor of primary importance in 
deciding whether attack is possible or not — has been shown in No. VIII 
of the present series to be influenced by the passive exosmosis of food 
substances from the host tissue into the infection drop. The present results 
show that, granted suitable conditions, a like effect can be produced through 
the action of volatile substances arising from the plant and accumulating in 
the atmosphere. A priori one would expect this latter effect to be least 
in the open field. On the other hand, one would anticipate that it would 
play a greater part under conditions of storage. Here the atmosphere is 
stagnant and volatile substances can accumulate to a degree comparable 
with the conditions which gave results in the present experiments. To cite 
particular instances, one may confidently state that the atmosphere of an 
apple store is very favourable to Botrytis germination ; and that the con- 
verse is true for the atmosphere of a potato store. 
As regards the effect of volatile constituents of the plant on the growth 
of the fungus, once the latter is inside, little can be said at present. 
Unpublished experiments have clearly shown that the capacity of crude 
unboiled plant extracts to cause germination and growth of fungal spores 
is in many cases very different from that of the boiled extracts. An 
extreme case of this' is afforded by onion juice, which inhibits Botrytis 
germination when in the unboiled condition, whereas when boiled it allows 
ready germination and growth. An investigation of this phenomenon is in 
progress and, though it is fairly clear that other factors also are concerned, 
the influence of volatile elements is being kept in view. 
