238 THE LARCH. 



to be the effect of tliat disease like mildew, to which 

 the larches, since 1802, have been subject; for though 

 that disease sometimes, and indeed often, kills them 

 in low, damp situations, yet, generally speaking, I know 

 of no instances sufficiently numerous to warrant me in 

 supposing that it has produced the effect which is the 

 subject of this letter. Where, therefore, the soil and 

 climate suit the larch, there cannot he a greater national 

 henefit than there to plant it. 



" In our endeavours to detect the source of a disease 

 which appears to attack forests and individual trees, 

 under the most various and indeed opposite circum- 

 stances, we ought, I think, to turn our attention to 

 what may be considered the proximate and remote 

 cause of the malady. The proximate cause is to be 

 sought in the physiological structure of the tree ; the 

 remote cause must be traced to the soil. In my first 

 letter to the Duke of Portland, I slightly adverted to 

 both these causes ; and I shall now cursorily retrace 

 what I then observed. 



" In trees properly so called — that is, in plants with 

 woody stems — there is an annual formation of two con- 

 centric layers ; the one being a layer of new wood 

 called the alburnum, the other a layer of new bark, 

 called the liher. The previously formed layers of the 

 wood are confined within, and covered by, the new 

 woody formation ; and, on the other hand, the new 

 bark is covered by the old. Thus the two annually 

 produced sappy layers, are always in juxtaposition, 

 while the old layers are carried farther and farther in 



