392 PROF. J. PERCIVAL ON ‘‘ SILVER-LEAF”’ DISEASE. 
that the middle lamella, or part of the cell-wall corresponding to 
it, has been dissolved or softened and then torn asunder. 
Various suggestions have been made in regard to the cause of 
the disease. Some authorities have hazarded the opinion that it 
is caused by bacteria, others that it is due to a deficiency of 
certain food-constituents in the soil. 
No spots or blisters are visible upon the leaves, and fungi are 
absent both from the affected leaves and the external and internal 
tissues of the stem and branches. 
In advanced cases a discoloration of the central parts of the 
wood is observable when the stems are cut across, and this 
sometimes extends to the wood of the finer branches, and into 
the wood of the leaf-petioles. In milder attacks, where the 
disease is of recent origin, the wood of the stems and branches is 
not visibly discoloured so far as the naked eye can determine, 
but microscopic examination often reveals brown stains on the 
walls of the vessels and meduilary-ray cells. 
When first noticed it is often confined to a few of the branches 
on one side of the tree, but sooner or later the whole tree becomes 
affected. 
Last September I dug up a fifteen-year-old plum-tree which 
had been subject to “silver-leaf” for several years. Many of 
the large roots were diseased. When cut across, the wood of 
these roots was seen to be discoloured, being dark brown instead 
of yellow as in the healthy wood. No rottenness or disintegration 
of tissue was observable; the discoloured parts were quite as 
hard as the uninjured tissues. 
The bark and cortex of the root covered the wood completely 
as in the healthy parts; and unless a transverse section had been 
made of the roots, the internal damage would scarcely have been 
suspected. 
Microscopic examination of the discoloured wood, especially 
where it bordered on the healthy tissues, revealed the presence 
of fine fungus-hyphe, which were traced for a considerable 
distance in the lumen of the vessels. Many of the vessels and 
medullary-ray cells were filled with a brownish-red substance, and 
the walls of these cells were stained brown. 
I sawed the thick diseased roots into one-inch lengths and 
placed them in a damp chamber. In two or three days a dense 
white mycelium of very delicate hyphe was developed upon each 
piece, round the edge of the diseased patch, as in fig. 5. 
