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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 alkali, is abundant in the canker of the elm. 
The old age of a tree, in this respect, 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 plaster of 
earth : but these methods, though they have been 
much extolled, probably do very little in producing 
a regeneration of the part. Perhaps the applica- 
tion of a weak acid to the canker might be of use ; 
or where the tree is of great value, it may be 
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 
operation of external elements, there are many 
others perhaps 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 attach 
themselves to trees and shrubs, feed on their juices, 
destroy their health, and finally their life, abound 
in all climates ; and are, perhaps, the most formi- 
