64 THE LARCH CANKER 
obscure. When a wound occurs in nature, it is covered in 
a very short time with a layer of resin, through which 
infection cannot take place, so that it is only a source of 
danger for a very brief period. But in an experiment, 
immediately a wound is made, spores or pieces of mycelium 
are thrust into it, and the resin which is secreted is poured 
outside the fungus, which can then attack the tissues inside. 
Contributory causes of canker. As has been known ever 
since Berkeley first investigated larch canker, the primary 
cause of the disease is Dasyscypha calycina. But there are 
many secondary or contributory causes, which while unable 
by themselves to bring about a canker, encourage the 
fungus, or in some way make the tree more susceptible to 
its attack. These factors are therefore of great importance, 
and the means adopted by foresters in their attempts to get 
rid of canker have been based much more on the mastery 
of secondary causes than of the primary cause, of which 
they are often ignorant. 
One class of contributory causes has already been dealt 
with in the section on ‘the mode of infection in nature’. 
This included all the factors which may cause wounds. 
A great deal of attention has formerly been paid to this 
class, and probably its importance has been over-estimated. 
Other contributory causes may be grouped under headings 
of climate (including altitude), soil, and mixtures of trees 
in plantations. 
We have here chiefly to rely on the published opinions 
of practical foresters, which may be found in articles and 
letters from time to time in the Gardener’s Chronicle and 
in answers to the English Arboricultural Society’s inquiry 
into the causes of larch canker (Somerville, 1895), and to 
the similar Scottish inquiry (Richardson, Borthwick, and 
Mackenzie, 1905). The best and most critical survey of the 
whole question is given by Forbes (1904), and as there is 
much diversity of opinion among foresters as to the causes 
in question, Forbes’s account is most valuable. 
Altitude and climate. From the time of Hartig (1880) 
onwards it has always been a matter of speculation why 
