248 



DISEASES OF TREES 



In the course of years the exposed wood dries up, and decay 

 begins to make its appearance, being greatly favoured by the 

 larvae of Sirex, which bore from the surface of the wound deep 

 into the wood, and thus enable rain-water to reach the interior of 

 the tree. Decomposition frequently spreads from the wound 

 high up the tree, and does so much damage that in woods where 

 resin is collected the yield of timber may be reduced from seventy 



FlG. 142. Transverse section of the stem of a spruce which has been tapped for 

 resin on four sides for ten to fifteen years. The only wood that is capable of 

 conducting water is those portions of alburnum, a, which are marked ofif by lines 

 between the four gutters. The wood, /;, beneath the two upper gutters is much 

 decomposed, whereas the wood, c, beneath the other gutters has remained sound. 

 Numerous galleries, e, formed by Sirex are seen proceeding from the upper 

 gutters. One fifth natural size. 



to twenty or thirty per cent, of the gross output It has 

 not been proved that trees that are tapped suffer in growth, 

 nor is it to be expected that such will be the case, seeing that 

 trees cannot utilize turpentine for growth. Tapping, however 

 greatly reduces the value of timber, because the quality to 

 a large extent depends on the amount of resin which it 

 contains. 



