THE CULTIVATION OF THE OAK 157 
which germinate on the bark, but cannot infect the tree 
unless there is a wound in the neighbourhood. How- 
ever, owing to the numerous small cracks and ruptures 
due to the injuries caused by insects, hail, frost, &c., 
the mycelium easily gains access to the cortex and cam- 
bium, and feeds on the contents of the cambium cells, 
which it destroys. The consequence of the irritations 
set up is the formation of canker-like knots on the 
branches, and the injury may be great enough to destroy 
smaller ones, and occasionally even a large one. 
Unquestionably the most important of the diseases 
to which the older oak-trees are subject are those which 
result in the destruction of the timber. 
There are about six or eight of the fungi known popu- 
larly as toadstools—technically as Hymenomycetes—which 
are able to injure and even destroy the timber of stand- 
ing oaks, and while each of these pests does the damage 
in its own peculiar way, they show considerable simi- 
larity in general behaviour. In the first place, these 
fungi are unable to penetrate the bark of sound trees, 
and their hyphe always gain access to the timber by 
means of actual wounds and exposed surfaces of wood, 
such as the cracks caused by frost or by the bending 
down of heavy branches under the weight of a load of 
snow, or the ruptured ends of broken branches blown 
off by strong gales or struck by falling trees, or places 
where animals have removed the bark, where cart-wheels 
have abraded the larger roots, and so on. Once inside, 
the hyphee of these fungi pierce the vessels, cells, &c., of 
