DEFECTS AND UXSOUXDNESS. 



63 



should a number of annual zones of wood cover it, the value 

 of the timber may be only slightly impaired. This is frequently 

 the case with conifers, where the slight crack which remains in 

 the interior of the stem becomes filled with resin, preventing any 

 decay of the wood. 



Very frequently, however, and especially in the case of broad- 

 leaved trees, frost-cracks which are only superficially occluded 

 are opened again and again in the winter, year after year ; the 

 occluding wood then gives rise to a prominent ridge, termed a 



frost-rib (figs. 8 and 9), and the wood then becomes unsuitable 

 as timber. These frost-ribs are developed in a very marked way 

 on the north side of young elms and oaks [but may occur on 

 any side of a tree. — Tr.] 



The great damage done by frost-cracks may be readily 

 observed on the transverse section of many old standard trees, 

 as in fig. 10. It is clear that such damage must introduce decay 

 into the wood and render it useless except for fuel. 



Frost-cracks are more frequent in the case of large and isolated 

 trees than in crowded woods ; they also commonly occur at places 

 in stems where the density of the woody texture varies, as at the 

 base of a tree and the! junction of broken or dead branches 

 with the stem. Trees which split easily, and those with large 

 medullary rays, are also very subject to this defect. 



Of woody species, oaks, sweet chestnut, limes, horse-chestnut, 



