FORMATION OF WOOD 271 



pass from one part of the stem to another, they 

 also act as storage tissue and are invariably 

 rich in carbohydrates. The size and number 

 of these rays in cross section are invariably 

 characteristic of the woods to which they belong. 



" The elements of the wood are generally 

 parallel in direction to the axis of the stem or 

 limb in which they occur — i.e. the wood is 

 straight - grained ; but they may be spirally 

 twisted round the stem, or obhque, in which 

 latter case, if successive layers lie 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 (Figs. 74-6), frequent in Maple ; whilst slight 

 projections or depressions repeated on the outer 

 surface of successive annual layers produce the 

 bird's-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 of mites, causes similar orna- 

 mental wavings of the grain. 



" One main cause of the elements not being 

 vertical is their growth in length and in dia- 

 meter 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 



