PROF. J. PERCIVAL ON ‘* SILVER-LEAF ” DISEASE. 395 
Although there is often little or no visible difference between 
the wood and other tissues of a sound branch and one on which 
“silvery” leaves are produced, I found that a diseased branch 
gives to water a faint brown tint when its end is placed in the 
latter for 24 hours, which is not the case with healthy branches. 
The tinted water gives a bright blue colour immediately with 
guaiacum and hydrogen peroxide, due to the presence of an 
oxidase. An oxidizing ferment exists also in the bast and cortex 
of healthy branches of Prunus species, but it does not leak out 
when the ends of such branches are dipped in water, although it 
dissolves out fairly readily when slices of these tissues are soaked. 
By cutting thin successive longitudinal slices from the branches 
of diseased plum-trees, from the bark inwards into the wood, 
and soaking these separately in water, it is seen that not only is 
the oxidase present in the cortex and bast in a very soluble 
condition, but it is also present in considerable quantity in the 
wood of the branch, which is not the case in healthy trees. 
From the evidence obtained during the examination of many 
cases, | have no doubt that the infection takes place below ground, 
no sign of any wound being visible above, and the internal 
damaged wood in which the fungus-hyphe are detected is 
generally confined to the root or the lower portion of the stem 
within the soil. 
In some instances, perhaps the majority, there were old 
partially occluded wounds through which the iofection might 
have taken place; but in others no abraded surface was noticeable, 
and the fungus appeared to have entered through the unwounded 
surfaces of the roots. Where horizontally placed roots contained 
the fungus, it was usually on the damp side facing downwards 
that the parasite had apparently entered, for this side of the 
root showed damaged woud and cortex when sawn across. 
Whether other species of Stereum are capable of setting up the 
disease, and the relationship of the superabuudant oxidase in the 
diseased branches to the peculiar changes in the epidermal cells, 
are matters which I hope to investigate later, when more material 
is available. 
The biology of Stereum hirsutum has been investigated by 
Prof. Marshall Ward, who has published his results in tie 
‘Philosophical Transactions’ of the Royal Society, Series B, 
Vol. 189 (1897), pp. 128-134, pls. 17-21. 
