18 THE LARCH CANKER 
are starved. Thus the roots die before the upper part of 
the tree, and the tree subsequently has all the appearance 
of having died from a root disease. Similar girdling may 
be seen in the side-branches of older trees, since these also 
grow very slowly in thickness. But when a main trunk is 
attacked in a portion which is more than four or five years 
old, the annual growth in girth is usually sufficient to 
confine the canker to one side. Also the healthy cambium 
at the back of a canker is more active than it is above or 
below, and the annual rings are consequently especially 
broad, so that a swelling is formed at the back of a canker 
which prevents the water-current of the tree being appre- 
ciably interrupted at this point (see figs. 1 and 9). There is 
thus no reason why the top of such a tree should not go on 
growing just as vigorously as one which has no canker ; 
and such is the case, for often in a twenty-year-old plantation 
the tallest and strongest-growing trees are found to be 
cankered near the base. 
One canker in an otherwise sound tree may not prove 
a very serious blemish; but when, as is not infrequent, we 
find as many as six or eight cankers on the main trunks of 
nearly every tree in a young plantation, then the value of 
each tree is reduced to a very small figure, and the wood is 
certain to prove a financial failure. It is against these 
attacks in which canker becomes epidemic that we have to 
protect our forests. The disease would not be so notorious 
were it not so extraordinarily hard to prevent. Only the 
forester, who, over acres of otherwise healthy larch planta- 
tions, sees canker after canker appearing, on his best trees 
as well as on his worst, can know what a curse this pest has 
become to European forestry. 
Sometimes a tree apparently recovers from a canker 
(fig. 8 and fig. 25). This only happens when the cork layers, 
by which the tree always tries to prevent the spread of the 
fungus, have been successful, and failing to find new feeding 
grounds the fungus has died of starvation. The surrounding 
tissues then grow over and occlude the canker just as they 
occlude the wound formed by the fall of a branch. It may 
