THE LARCH CANKER 17 



apothecium has a short white stalk. When growing in this 

 way (i. e. saprophytically) it does no damage to the tree. 

 It is only when it begins to live parasitically and to prey 

 on the living tissues that a canker is formed. 



When it has penetrated the cork protection of a tree, the 

 mycelium of the fungus kills the cortex and phloem (the 

 soft tissues which surround the wood) and causes them to 

 turn brown. Next it attacks the cambium, and since this 

 is the tissue which gives rise to new layers of wood and 

 phloem, its death renders the tree incapable of further 

 growth in thickness- at this point. The fungus spreads very 

 slowly into the surrounding living tissues, and gradually 

 kills a wider and wider area of cambium, so that a larger 

 patch of the tree each year fails to keep up with the growth 

 of the rest. In this way a flattened cavity is formed at the 

 point where the cambium has been killed, the dead bark 

 becomes blackened, resin oozes out and streams down the 

 tree, and the whole presents the ugly appearance of a black 

 blister or running sore. This is the canker. 



On the blister may often be fgund the fungus fructifica- 

 tions, which are generally larger than the similar apothecia 

 on the dead branches. When fresh these are orange -red 

 in colour, but later they become bleached to a dirty white 

 or yellow, and in this state may be preserved for many 

 years in a stream of resin. 



The effect which canker has on a tree depends on the age 

 at which the attack occurs. To young trees it is often 

 fatal, for the trunks grow but slowly during their first five 

 or six years, and may consequently be entirely girdled by 

 the fungus. This very frequently happens when grass and 

 weeds are allowed to grow long among recently planted-out 

 trees, producing humid conditions which encourage canker 

 formation. When a tree is girdled in this way the upper 

 part continues to grow for some time, for it still obtains 

 water and mineral food from the soil and carbon from the 

 air ; but organic compounds which it sends down towards 

 the roots are checked at the point of girdling, causing 

 increased growth and bulging just above it, and the roots 



1888 /i 



