DISEASES. 241 



to what agent or original cause shall we ascribe that 

 paralysis, or want of due conducting power in the 

 vessels of supply? Are we to attribute it to some 

 natural debility in the constitution of the tree, or to 

 some external exciting stimulant, producing morbid 

 action in the secreting vessels at a certain period of 

 the tree's existence ? Most writers who have noticed 

 the decay of larch timber are inclined to ascribe it to 

 a specifically unnutritive, that is, an inappropriate 

 soil. Thus, in a letter from Dunkeld, a copy of which 

 has just been handed to me, the writer observes — ' I 

 am sorry to inform you, that in many situations the 

 larch is decaying here before it arrives to a large size, 

 and more especially in moist situations. The larch 

 you know to be a thorough Alpine tree, and therefore 

 dry subsoil ought to be chosen. Crags and rocks, 

 where there is barely soil sufficient for the nourish- 

 ment of any other tree, are places suited to the larch, 

 and where it will grow to be a giant forest-tree, and 

 as sound and compact as boxwood.' ' Oak timber,' 

 Mr. Knight says, ' is found to decay most rapidly, gene- 

 rally when it has been grown in soil of small depth ; and 

 to some defect of the soil the decay of the lairJi timber 

 must, I conceive, be attributed. A ivet soil, by destroy- 

 ing all but the superficial roots, may, I think, act like 

 a light or shallow soil. The larch shows no disposition 

 whatever to decay when planted in deep argillaceous 

 loam, and I have not even seen it decay in this country 

 in the manner described by the Duke of Portland.' 

 A dryness of soil, then, however writers may differ in 



