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, 

 however, 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. 8, 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 neighbor. Each cone ordinarily 

 constitutes a year's growth, and therefore there 

 is a greater number of them at the base of a tree 

 than higher up. These cones vary greatly in 

 Uiickncss, or, looking at a cross-section, the rings 

 vary in width; in general, those at the center 

 being thicker than those toward the bark. Va- 

 riations from year to year may also be noticed, 

 showing that the tree was well nourished one 

 year and poorly nourished another year. Rings, 

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

 "False rings" are sometimes formed by a cessa- 

 tion in the growth due to drouth, fire or other 

 accident, followed by renewed 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 these 

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

 they were cut up thru the center, as in Fig. 9. 



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

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

 and give the charactistic pleasing "grain" of wood. Fig. 27, p. 35. 

 The annual rings can sometimes be discerned in the bark as well as 

 in the wood, as in corks, which are made of the outer bark of the 

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



The growth 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. 9. Diagram of 

 Radial Section of 

 Log- (exaggerated) 

 Showing Annual 

 Cones of Growth. 



