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-growdng 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 hajDpens when the cork layers, 

 by which the tree always tries to prevent the spread of the 

 fungus, have been successful, and faihng 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 



