182 SECTIONAL ADDRESSES 
rind is very resistant to invasion. If these moulds are grown in orange 
juice they produce substances, perhaps enzymic, which cause the pectic 
decomposition of the outer rind, thereby destroying its resistance to 
attack. In this way the spread of rotting from one mouldy orange to 
sound fruits in contact with it can be accounted for. On the other hand, 
if these fungi are grown on the usual synthetic media they are incapable 
of producing this resistance-destroying system, as Green terms it. This 
is an interesting illustration of the fact that the biochemistry of micro- 
organisms varies considerably according to the nature of the substratum 
on which they grow. 
Another branch of plant pathology which is receiving much attention 
at the present time and which will probably assume greater importance in 
the future is the influence of one micro-organism on another in the estab- 
lishment of disease. In ecology generally the factors of competition and 
the influence of one plant on another have long been held to be of supreme 
importance. The effect of such interactions is now receiving attention 
from plant pathologists, and the judicial consideration of factors of this 
kind will lead to a better ecological interpretation of the incidence of 
disease in plants. Fawcett *! has already stressed the importance of this 
aspect of plant pathology. 
The effects of associations of micro-organisms in culture are often pro- 
found. A mixed culture of two organisms can often produce a result 
which neither of them alone can induce. Such an effect has been termed 
‘synergism’ by Holman and Meekison.42 One organism may change 
the substratum so that it becomes suitable for the growth of the other, 
but the interactions of the two associates on the original medium are some- 
times of a more intricate nature. For associations of bacteria the literature 
has been reviewed by Buchanan and Fulmer,* and this has been done 
to some extent for fungi by Harder,** Porter 4° and Machacek.*® The 
sequence of fungi on natural substrata is related to such associations. On 
a tree log, for instance, a succession of different fungi usually develops in 
orderly sequence over a period, one species apparently preparing the way 
for another. An analysis of the factors which determine such a succession 
would be of great interest. The effects of combinations of specific yeasts 
and bacteria are illustrated by the fermentation processes induced by 
the ginger-beer plant and by kephir grains. ‘The influences of micro- 
organisms on one another in culture may be very diverse: instead of 
producing an effect which neither alone can bring about, one organism may 
greatly stimulate the activity of the other or may completely inhibit it, 
or the two organisms may be entirely indifferent to one another. In 
Nature associations of diverse micro-organisms are the rule rather than 
the exception, so it is important for the microbiologist and the plant 
pathologist to study these complexes. 
“| Phytopathology, 21, p. 545 (1931). 
42 Jour. Infectious Diseases, 89, p. 145 (1926). 
Physiology and Biochemistry of Bacteria, vol. iii (1930). 
“4 Naturw. Zeitschr. Landw. Forstw., 9, p. 129 (1911). 
4° Amer, Jour. Bot., 11, p. 168 (1924). 
48 Macdonald College, McGill University, Tech. Bull. No. 7 (1928). 
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