GENERAL SUMMARY 187 



Some soils are unsuitable for tree growth owing to the 

 presence of a pan or hard stony layer, which sometimes 

 forms in sandy soils. When a pan is near the surface it 

 has to be broken through, in planting, to allow free develop- 

 ment for the roots. When at a depth of 12 in. or more it 

 may not be discovered during planting operations, but it 

 is none the less detrimental to tree growth, as it interferes 

 with -the normal flow of water. In wet weather it allows 

 water to stagnate round the tree roots, and in times of 

 drought it prevents water rising by capillary attraction 

 from the lower levels where moisture is still present. One 

 of the worst cases of canker that I have met with can only 

 be adequately explained by the trees being weakened 

 through shortage of water caused by such a pan in dry 

 weather. 



It has been found that the correct choice of site, soil, 

 and mixture does much to restrict attack by the canker 

 fungus. This is not because under these circumstances the 

 canker fungus is less common, for, wherever larch is grown, 

 it is present on the dead branches in sufficient profusion to 

 infect all the trees. It is the direct outcome of the greater 

 vigour of the trees. The larch normally protects itself 

 against the canker, first by its original layer of cork, which 

 covers the whole tree and extends across the phloem and 

 cortex of newly dead branches, and, if the fungus has passed 

 this, by new layers of cork which are put in to prevent its 

 further penetration. If the tree is vigorous these layers 

 are made quickly and made thick, so that the fungus has 

 much more difficulty in reaching the cambium. By such 

 means the tree throws off attacks and cankers are pre- 

 vented ; but if the trees are weakly, the fungus has things 

 too much its own way, and has little difficulty in reaching 

 the cambium and forming cankers. 



It has been shown that the most frequent way in which 

 the main stems of larch trees become infected with canker 

 is by the fungus, which lives saprophytically on the branches, 

 growing down, past the cork layers at the branch-bases, into 

 the trunk. It is suggested that when larch woods are 



