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and it is invariably connected with old age. The cause 

 seems to be an excess of alkaline and earthy matter 

 in the descending sap. I have often found carbonate 

 of lime on the edges of the canker in apple trees; and 

 ulmin, which contains fixed alfcili, is abundant in the 

 canker of the elm. The old age of a tree, in this res- 

 pect, is faintly analogous to the old age of animals, in 

 which the secretions of solid bony matter are always in 

 excess, and the tendency to ossification great. 



The common modes of attempting to cure the 

 canker, are by cutting the edges of the bark, binding 

 new bark upon it, or laying on a plaister of earth; but 

 these methods, though they have been much extolled, 

 probably do very little in producing a regeneration of 

 the part. Perhaps the application of a weak acid to 

 the canker might be of use; or where the tree is of 

 great value, it maybe watered occasionally with a very 

 diluted acid. The alkaline and earthy nature of the 

 morbid secretion warrants the trial; but circumstances 

 that cannot be foreseen may occur to interfere with the 

 success of the experiment. 



Besides the diseases having their source in the 

 constitution of the plant, or in the unfavourable opera- 

 tion of external elements, there are many others per- 

 haps more injurious, depending upon the operations 

 and powers of other living beings; and such are the 

 most difficult to cure, and the most destructive to the 

 labours of the husbandman. 



Parasitical plants of different species which at- 

 tach themselves to trees and shrubs, feed on their 

 juices, destroy their health., and finally their life, 



