THE LARCH CANKER 63 



trees artificially unless they were first wounded in some way. 

 But infection experiments may be incapable of demonstrating 

 a phenomenon which is almost ubiquitous in nature, and 

 the case of larch canker is one in which it is exceedingly 

 difficult to repeat in the culture house a process which we 

 can observe in the forest. For every time the mycelium 

 successfully grows from a dead branch to the trunk, it 

 must fail at least a dozen times. But experiments which 

 at the luckiest were only successful in 8 per cent, of the 

 inoculations would not by themselves be conclusive, espe- 

 cially when we know that minute punctures, too small to 

 be seen with a simple lens, may be sufficient to admit the 

 fungus, and these might have been overlooked in the experi- 

 ment. It is thus by no means easy to get satisfactory 

 results with experiments set up to show that Dasyscypha 

 can infect living trunks from dead branches, and if such 

 experiments did apparently succeed, it is difficult to be 

 quite certain that the fungus has not entered in some other 

 way. It is for these reasons that the arguments in favour 

 of this means of infection have been drawn, not from culture 

 experiments, but from observation. 



Conversely, wounds which have been found necessary for 

 artificial inoculation are probably of much less importance 

 in the forest. I have often seen billhook wounds which 

 have healed naturally even in plantations where canker was 

 epidemic. And where shooting rides have been cut in larch 

 woods there is not generally any increase in the frequency 

 of the fungus. Again, when larches are grown as nurses 

 for other trees, the branches have regularly to be cut back 

 to make room for the main crop, and yet, in such cases, 

 cankers are often less numerous than in other plantations 

 in which there has been no pruning ; indeed, in a plot 

 treated in this way in Bagley Wood near Oxford, where 

 larch was grown as nurses for deodar, there was no canker 

 at all on these nurse trees, whereas a larch plot less than 

 200 yds. away was attacked with moderate severity. 



The reason why laboratory experiments give a false 

 impression of the importance of wounds is by no means 



