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 hyphee 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 hypha3 of these fungi pierce the vessels, cells, &c., of 



