250 DISEASES OF TREES 



die. The dying twigs and branches are more or less quickly 

 decomposed by saprophytic fungi. 



The rate of decomposition and the period when the branches 

 will drop off are most of all regulated by the condition of the 

 wood. Branches of dicotyledons which consist only of alburnum 

 drop off much sooner than branches which contain duramen. 

 On account of the shaded branches of young Scotch pines 

 consisting of soft broad-ringed wood, these trees clean them- 

 selves much sooner than the spruce and silver fir, the wood 

 of whose branches is tough, firm, and durable. The thicker, 

 more resinous, and narrower-ringed branches on the upper part of 

 the stem of the Scotch pine, on the other hand, retain their 

 position for a long time, and are more or less embraced or over- 

 grown at their bases by the growth of the stem. This embracing 

 of dead branches is the general rule in the case of the silver fir 

 and spruce, and as they have no organic connection with the 

 adjoining wood-layers they drop out of boards as loose knots 

 when the wood shrinks in drying. 



The embracing of dead branches would be a much commoner 

 occurrence, were it not for the fact that the base does not die, 

 and in the case of the thicker branches it often remains alive for 

 a distance of about two inches (Fig. 143). The base of the 

 branch, being nourished from the stem, remains alive, and is 

 capable of growing in thickness. When, after some years, the 

 increase in thickness of the bole of the tree has become equal to 

 the length of the living basal part of the branch, the dead part 

 of the branch will have become so much decayed as to drop 

 off under the action of wind, snow, &c. (Fig. 144). After the 

 wound has healed over only a small dark brown blotch remains 

 in the interior of the tree to indicate the limits of the enclosed 

 stump. 



It is in the manner just described that the tree protects itself 

 against the dead stumps of branches being overgrown. It is 

 only the larger branches that frequently do not drop off until a 

 portion of the dead base has been embraced by the stem. In 

 the case of conifers this portion is saturated with resin, and in 

 the case of dicotyledons it is more or less decomposed. After- 

 wards, when the branch has become completely rotten and has 

 dropped off, a hole remains behind which is only partially filled 



