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 Dasijscypha 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 cgiuses 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 cHmate (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 Enghsh Arboricultural Society's inquiry 

 into the causes of larch canker (Somcrville, 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 



