3*4 
action of the atmosphere, more especially owing to the presence ot 
the carbon dioxide in it; (2) that the insoluble compounds are dis- 
solved by some substance or substances secreted by the sprayed 
leaves ; (3) that the copper is rendered soluble by some substance 
secreted by the fungus itself, which consequently brings about its own 
destruction. 
The first of these theories was supported by Pickering (see 
Eleventh Report on the Woburn Experiment nl Fruit Farm, 1910) ; but 
recently, further work carried out by Gimingham, and by Barker and 
Gimingham, discredits this theory and lends support to the last, 
namely that of the action of the fungus itself. (Journal of Agricul- 
tural Science , Vol. IV, pp. 69 and 76. 
Pickering found that the insoluble substances containing copper 
formed in the preparation of Bordeaux mixture were partly dissolved 
in water containing a large amount of carbon dioxide and that copper 
sulphate was formed in the solution. This naturally led to the idea 
that the fungicidal action of the mixture was due to the effect of 
atmospheric carbon dioxide on the insoluble copper precipitate. 
Furthermore, he observed that when the mixture contained excess of 
lime, no copper appeared in the solution until all the lime had been 
converted into chalk by the action of the carbon dioxide. Conse- 
quently, he recommended that care should be taken to prepare 
Bordeaux mixture without excess of lime, since this substance would 
only delay the action of the mixture as a fungicide. 
Gimingham, however, found that if the excess of carbon dioxide 
was removed from the liquid, the copper was again precipitated in an 
insoluble form ; while as the result of several experiments he finally 
concluded that it was unlikely that the copper was rendered soluble 
by the action of the atmosphere. 
The possibility that the copper is rendered soluble by secretions 
from the sprayed leaves was examined by Barker and Gimingham. 
They found that a certain amount of soluble copper sulphate was 
produced by the substances secreted through minute punctures or 
abrasions on the surfaces of the leaves. This quantity, however, was 
not sufficient to account for the fungicidal action of the Bordeaux 
mixture; on the other hand, it was enough to cause scorching of the 
leaves in spots, and explains why older leaves which h^ve been longer 
subject to possible damage are more liable to scorching than younger 
foliage, since undamaged leaves do not appear to secret the necessary 
substances. 
The same workers then turned their attention to the possibility 
that the copper is rendered soluble by substances secreted by the 
spores or germ tubes of the fungi themselves. They found that spores 
possessing thin walls, and also the tips of young germ tubes, do 
actually appear to secret small quantities of substances sufficient in 
amount to dissolve enough copper to cause their death. The amount 
of copper dissolved depended, however, on the distance of the spore 
