THE LARCH CANKER 07 



of recently cleared coniferous woods, the superficial layer 

 is poor in nutriment ; and if the site is one where grass or 

 heather has recently been groAving, the roots of these plants 

 will prevent a free interchange of gases between the soil 

 and the air. Larch is peculiarly sensitive to the condition 

 of the surface, for being in its youth a surface-rooter it 

 cannot range through the deeper layers in search of the 

 nutriment which it requires. For the same reason the sub- 

 soil is of secondary importance, i. e. for the normal growth 

 of the tree. (The nature of the subsoil is an all-important 

 factor in susceptibility to heart-rot.) So long as this is 

 well drained a good thick surface-soil will grow satisfactory 

 larch, and if the trees are growing vigorously there is a good 

 chance of their remaining free from canker. Perhaps the 

 safest situation is a mountain-side, where the porous, well- 

 drained, gravelly soil derived from the rocks above provides 

 all the factors favourable to larch culture. 



Only in comparatively few cases can we say that a soil 

 will or will not grow healthy larch until it has been tried. 

 I have seen healthy woods grown on a clay subsoil and 

 plantations riddled with canker on sand. Mitchie (1885) 

 says that soil which is suitable for barley will generally 

 prove successful with larch. 



Conditions of culture. Pure larch -plantations are more 



liable to attack than trees which are mixed sparingly with 



hardwoods, such as hornbeam or beech. This is usually 



explained by supposing that the hardwoods act as a kind 



of screen, protecting the larch from fungal spores which are 



blown through the woods. Such a theory, however, cannot 



be entertained when, as is often the case even in mixed 



woods, the dead lateral branches of the larch become 



covered with the fructifications of DasyscypM. For this 



shows that the spores have found out the larch trees, and 



the number of fructifications made on the dead branches 



would be sufficient to ensure infection of the trees, were 



they susceptible. Probably Forbes is right in accounting 



for the comparative immunity in mixed woods in quite 



another way. 



F 2 



