258 
BRYCE : 
The wound thus produced is closed over in a manner common 
to trees in general, i.e., the living cambium cells surrounding 
the wound undergo rapid division and give rise to a callus or 
cushion of tissue which grows over the wound area and 
ultimately covers up the wound. A swelling is thus formed 
over the site of the wound, but this gradually disappears in 
the subsequent growth in thickness of the stem. Thus, 
tapping is not interfered with permanently, though care is 
necessary when again tapping over this point to avoid grazing 
the woody swelling on the stem wood, if it is not yet merged 
in the subsequent growth. This healing process is known as 
“ occlusion,” and is a method whereby the tree covers up 
exposed wood areas and re-unites the severed edges of the 
cambial layer, so that one continuous cambium is again 
formed, and the stem can continue its normal growth in thick- 
ness. The new wood, however, never unites across the wound 
with the old wood, and in sections across the stem the wound 
is always visible. The cause of this kind of swelling being 
known, measures can be adopted to avoid producing it. 
The second kind of swelling is due to the production of 
nodules. About the cause of the production of nodules much 
difference of opinion prevails ; several explanations have been 
advanced, but none so far has found general acceptance. A 
nodule at first is a little isolated body of woody tissue 
lying in the cortex, usually about the size of a “ pea ” when 
first observed, and easily “ shelled out ” with a penknife. 
There is little to indicate its presence at this stage — occasion- 
ally a small protuberance, or a slight cracking of the bark 
externally. In later stages these “ peas ” increase to the size 
of a “ hen’s egg,” or many “ peas ” fuse together and form an 
irregular mass ; or, again, large sheets of woody tissue are 
produced. At the same time growing points originate, which 
grow inwards and unite with the stem wood, and thus ultimately 
the nodular masses become connected with the stem wood at 
many points. As the nodules grow larger the stem becomes 
gnarled ; the cortex cracks and latex oozes out ; finally, the 
entire stem to a height of 5 or 6 feet from the ground is affected. 
In this condition it is impossible to carry on tapping, and the 
tree is useless. 
