THE STRUCTURE OF WOOD. 



19 



As a rule heart-wood is more valuable for timber, being harder, 

 heavier, and drier than sap-wood. In woods like hickory and ash, 

 howevei', which are used for purposes that require pliability, as in 

 baskets, or elasticity as in handles of rakes and hoes, sap-wood is 

 more valuable than heart-wood. 



In a transverse section of a conifer, for example Douglas spruce. 

 Fig. S. the wood is seen to lie in concentric rings, the outer part of 

 the ring being darker in color than the inner part. In reality each 

 of these rings is a .section of an irregular hollow cone, each cone en- 

 veloping its inner ncighlior. Each cone ordinarily 

 constitutes a year's groi\'tli, and therefore there 

 is a greater number of tliem at the liase of a tree 

 than higher up. These cones vary greatly in 

 //nV'/rH ('.!-■,?. or, looking at a cross-section, tlie rings 

 vary in ividlJi ; in general, those at tlie center 

 being thicker than those toward the bark. Va- 

 riations from year to year may also l:>e noticed, 

 showing that the tree was well n(jurished one 

 year and pjoorly nourished another year. ]!ings, 

 however, do not always indicate a year's growtli. 

 "False rings" are sometimes formed by a cessa- 

 tion in the growth due to drouth, fire or other 

 accident, followed by rene^^•cd growth the same 

 season. 



In a radial section of a log, Fig. 8, these 

 "rings" appear as a series of parallel lines and 

 if one could examine a long enough log tliese 



lines would converge, as would tlie cut edges in a nest of cones, if 

 they were cut up tliru tlie center, as in Fig. 0. 



In a tangential section, tlie lines appear as broad bands, and 

 since almost no tree grows perfectly straight, these lines are wavy, 

 and give the cliaractistic pdeasing "grain" of wood. Fig. 27, p. 35. 

 The annual rings can sometimes be discerned in tlie hark as well as 

 in the wood, as in corks, whicli are made of the ontei' bark of the 

 cork oak, a product of soutliern Europe and northern Africa. Fig. 10. 



The gTowth of the wood of exogenous trees takes place thru the 

 ability, already noted, of protoplasmic cells to divide. The cambium 

 cells, which have very thin walls, are rectangular in shape, broader 

 tangentially than radially, and tapering above and below to a chisel 



Fig". 'I. Diayram of 

 Radial Section of 

 Log" (exaggerated) 

 Showing Annual 

 Cones of (xrowth. 



