184 SECTIONAL ADDRESSES 
play an important part in the specificity of saprophytism exhibited by 
certain fungi. The first fungus to appear on a newly exposed oak stump 
is commonly Stereum hirsutum whereas on a birch stump S. purpureum 
takes this position. In the laboratory these fungi can be grown in culture 
both on oak and on birch wood. Both fungi are widespread and their 
spores are probably equally abundant in the air so that their chances of 
alighting on oak and birch stumps are similar. Allowing that oak wood 
is initially slightly more favourable for the growth of S. hirsutum than for 
S. purpureum, at a later stage the permeation of the wood already by 
S. hirsutum probably prevents the development of S. purpureum. As 
previously indicated the occupation of exposed wood in plum trees by 
comparatively harmless micro-organisms tends to prevent subsequent 
invasion by the dangerous Stereum purpureum. 
In plant pathology there is now a large mass of data concerning the 
inhibiting effect sometimes evident of one organism on another. In many 
instances the inhibiting organism is purely saprophytic, but more rarely 
two pathogenic organisms inhibit each other. Fawcett and Lee ** record 
that on inoculating the fungus Dothiorella gregaria and the bacterium 
Pseudomonas juglandis together into walnut branches no lesions were 
formed, although D. gregaria is parasitic on the branches and P. juglandis 
causes a blight of the leaves and young stems ; here the bacterium inhibited 
the pathogenicity of the fungus in the branches. Of particular interest 
and of great importance is the antagonism shewn by certain saprophytes 
to pathogenic fungi which invade the underground parts of their hosts : 
indeed it is not too much to say that a new chapter in soil microbiology 
has been opened with the recognition of this factor of biological antago- 
nism. In 1924 Porter °® shewed that inhibition of infection of wheat 
seedlings by Helminthosporium resulted when a certain bacterium as well 
as the fungus was introduced into the surrounding soil, and that there was 
considerable delay in the infection of flax seedlings by Fusarium Lint when 
the same bacterium was included in the soil. The manner in which such 
inhibition is brought about is unknown, but perhaps the bacterium in its 
growth produces some substance which is toxic to the fungus. In 1923 
Millard ®? demonstrated that Actinomyces scabies, a commonly occurring 
soil organism which causes potato Scab, could be prevented from attacking 
potatoes by incorporating large quantities of green manure in the soil. 
Millard and Taylor ** subsequently shewed that when another, purely 
saprophytic, species of Actinomyces, A. precox, was incorporated in the 
soil with A. scabies the latter was suppressed. ‘They suggest, therefore, 
that in green manuring for the control of potato Scab saprophytic species 
of Actinomyces, and perhaps also soil bacteria, are favoured to such an 
extent in competition with A. scabies that the latter is inhibited. More 
recent work on the antagonism of other soil micro-organisms to infection 
by pathogenic fungi in the soil has been ably reviewed by Garrett,*® but 
55 Citrus Diseases and their Control, p. 38 (1926). 
56 Amer. Jour. Bot., 11, p. 168 (1924). 
57 Aun. App. Biol., 10, p. 70 (1923). 
58 Ann. App. Biol., 14, p. 202 (1927). 
59 Biolog. Reviews, 9, p. 351 (1934). 
