30 TECHNICAL PROPERTIES OF WOOD. 



here ; thus all species, as a rule, form broader rings during 

 jouth than in old age. 



Owing to difterences in the breadth of annual zones in old 

 conifers, the weight <»f the wood frequently increases outwards; 

 hut in ring-pored, broad-leaved trees, and also in beech, the 

 opposite is the case. In young stems there is usually no 

 marked difference between the weight of the inner and outer 

 zones of wood. 



In cases of decay of the wood due to parasitic or saprophytic 

 fungi, the weight is always reduced ; this must influence the 

 relative weights of heartwood and sapwood. 



If we consider the respective weights of the upper and lower 

 parts of stems, much variety prevails, owing to the varying 

 conditions under which a tree has grown at difi"erent periods ; 

 in the majority of cases, it may be atiirmed that the lower part 

 of the stem is heaviest. 



It was found by Sanio and R. Hartig for the Scotch pine, and 

 by Exner for the beech, that, owing to the comparatively broad 

 zones of dense summer-wood below and spring-wood above, the 

 lower part of the stem contains heavier wood than its upper part: 

 the wood contained Avithin the crown becomes again heavier. 



Hartig also found that, with young oak trees up to 50 years 

 old, the weight of the wood increased upwards. With old and 

 very old oaks, on the contrary, the reverse happened, and this 

 was more frequently the case with trees growing in dense woods 

 than those in the open. 



For the birch, R. Hartig made the interesting discovery that 

 the weight of the wood was not affected by the breadth of the 

 rings, but by the age of the part on which the ring occurred ; 

 broad rings only appeared to contain less woody substance because 

 they belonged to the younger parts of the tree, and hence the 

 lower wood was heavier. 



Spruce and silver-fir grown in dense woods contain heavier 

 wood in the lower portion of their stems, but trees of these 

 species grown in the open, with crowns low down the stem, 

 contain heavier wood above than below. In the Scotch pine the 

 weight of the wood increases with its age, and is influenced by 

 the production in it of resin ; the heaviest wood is, therefore, 

 always in the lower part of the stem. 



