DISEASES. 225 



young state than the epidermis, and from the time it 

 ceases to cover and protect the young shoot, till such 

 time as the parenchyma becomes corky and strong, the 

 tree is exceedingly liable to injury from frost, cold, wet 

 and dull weather. The manner in which frost acts 

 most injuriously upon the young larch, besides scorching 

 the foliage, is by rupturing the globules of the sap- 

 vessels of the bark, which are fully surcharged with a 

 transparent greenish fluid, and when acted upon by frost, 

 cold, or wet, is changed into red or brown, when it may 

 be said the disease in question has commenced. On 

 carefully examining the liber of diseased trees with the 

 aid of the microscope, it is observable how many of the 

 cells are injured. The fluid in the healthy cells, as 

 already said, is of a transparent blue-green colour, and 

 in those affected or diseased, it is changed to a red or 

 reddish brown. The fluid when once changed never 

 regains its natural colour, and the diseased or injured 

 part never undergoes a complete healing. After the 

 parenchyma is fully formed and become corky, there 

 is little or no fear of the ulcer taking place. The 

 cortical bark entirely defends the tree against any 

 injury after it is fully formed, and I never saw a tree 

 seriously affected with blister after the bark had become 

 corky. If, therefore, warm and congenial seasons pre- 

 vail till the trees are about twenty years old, they are 

 then secure against being killed — at least if well clothed 

 with branches to near the ground. The forester is glad 

 when he sees the furrows and cortications of the bark 

 well developed, as then he knows the danger to the tree 



