ix.] HEALING OF WOUNDS BY OCCLUSION. 213 



ing, growing parts of the crown of leaves exposed to 

 sunlight and air above. 



The point I wish to insist upon here is that in these 

 cases of branch-breaking, however brought about, 

 open wounds are left exposed to all the vicissitudes of 

 the forest atmosphere ; if we compare the remnant of 

 such a broken branch and the scar left after the 

 natural shedding of a branch or leaf, the latter will 

 be found covered with an impervious layer of cork, a 

 tissue which keeps out damp, fungus-spores, &c., 

 effectually. 



It is, in fact as a matter of observation and experi- 

 ment these open wounds which expose the standing 

 timber to so many dangers from the attacks of 

 parasitic fungi ; and it will be instructive to look a 

 little more closely into the matter as bearing on the 

 question of the removal of large branches from trees. 



If a fairly large branch of a tree, such as the oak, is 

 cut off close to the trunk, a surface of wood is exposed, 

 surrounded by a thin ring of cambium and bark (as 

 in Figs. 27 and 28;. We have already seen what the 

 functions of the cambium are, and it will be observed 

 that the cut edge of the cambium (C) is suddenly 

 placed under different conditions from the usual ones ; 

 the chief change, and the only one we need notice at 

 present, is that the cambium in the neighbourhood of 



