38 OF WOOD IN GENERAL. 



in opposite directions the wood is cross-grained. A slightly wavy 

 longitudinal course in the elements of the wood produces the 

 condition known as curly grain, frequent in Maple ; whilst slight 

 projections or depressions repeated on the outer surface of 

 successive annual layers produce the birds-eye and landscape 

 varieties in the same wood. The presence of undeveloped buds 

 or knots, as in "burrs," produced on many trees by the attacks 



FIG. 28. Transverse section of Buckthorn (Khdmnus cathdrticus), showing 

 flame-like groups of vessels. 



of mites (Phytdptus), causes similar ornamental wavings of the 

 grain 



One main cause of the elements not being vertical is their 

 growth in length and in diameter after leaving the cambium 

 stage. Such growth in length causes the tips of the fibres to 

 crowd in between those above and below, and become interlaced 

 and oblique in direction. This adds to the toughness of the 

 wood and makes it less easy to split, and may produce a visible 

 twisting of stems or branches. 



