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 found 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 are! 
