﻿BRANCH CANKERS OF RHODODENDRON. 



79 



breaks off close to the trunk or even within the bark layer of 

 the trunk. A wound of this character then heals over rapidly. 

 The same is true of trees like the beech, poplar and other 

 hardwoods. In the case of the spruces, on the other hand, 

 the base of the branch stub keeps on growing, and spruce 

 trees are usually found with large numbers of dead branches 

 extending out from the main trunk, and when these finally 

 do break off, they break off two or three inches out from the 

 trunk, and the healing, instead of taking place even with the 

 surface of the trunk, takes place at a considerable distance 

 out at the apex of a well marked cone. 



The character of the healing is furthermore yeij consider- 

 ably influenced by the rate of growth of the tree species. A 

 rapidly growing tree will heal over branches very much more 

 quickly than a slow growing tree. 



In the case of the rhododendron cankers, one finds that 

 when a lateral branch dies or where the leading shoot dies, 

 the wood of the parent branch or trunk im- 

 mediately surrounding the base of the dead 

 branch likewise dies, and frequently for a 

 very considerable distance from the base of 

 the branch (see figure). The healing layer 

 then starts to form at a considerable dis- 

 tance away from the base of the dead 

 branch. The dead branches of rhododen- 

 dron are vcrj' persistent; i.e., they break 

 off in such a manner that the dead stub is 

 usually an inch or more in length. The rate 

 of growth of the rhododendron is extremely 

 slow, and it therefore takes a very long time 

 for a branch stub to be completely healed 

 over; so long, in fact, does this take that in the majority of 

 instances the dead branch stub has begun to rot away long 

 before the parent branch has succeeded in covering the stub. 

 Figures 3 and 4, plate 5, show two of these branch stubs, both 

 of which are gradually decaying. Owing to the slow growth 

 of the rhododendron wood, the callous lips increase in size 

 from year to year and gradually give rise to a small knot or 

 swelling, (figure 3, plate 6), which increases in size as the 



SECTION OF 

 OLD CANKER. 



