IX.] HEALING OF WOUNDS BY OCCLUSION. 221 



trees. All its tissues dry up, and its cortex, cambium, 

 &c., are rapidly destroyed by saprophytic fungi, and 

 in a short time we find only a hard, dry, branched 

 stick projecting from the tree. At the extreme base, 



Fig. 33— Base of a strong branch which had perished naturally twenty-four years 

 previously to the stage figured. The branch decayed, and the base was gradually 

 occluded by the thickening layers of the stem : the fall of the rotting branch did 

 not occur till six years ago, however, and can be determined from the layers at e 

 and/, which then began to turn inwards over the stump. Meanwhile, the base 

 had become hollow and full of rotten wood, g. It is interesting to note how slight 

 the growth is on the lower side of the branch base, z, as compared with that at h 

 above : the line numbered 24 refers to the annual zones in each case. As seen at 

 b and d, the rotting of the wood pas-^es backwards, and may invade the previously 

 healthy wood for some distance. (After Hartig.) 



where it joins the tree, the tissues do not at once 

 perish, but for a length of from half an inch to an 

 inch or so the base is still nourished by the trunk. 

 After a time, the wind, or a falling branch, or the 



