WOOD STRUCTURE 167 



you account for this ? Is each separate ring of uniform 

 thickness all the way round? Mention some of the cir- 

 cumstances that might cause a tree to grow less on one 

 side than on the other ; such, for instance, as too great 

 shading, lack of foliage development from one cause or 

 another, exposure of roots by denudation, etc. Are the 

 rings of the same thickness in all kinds of wood ? Which 

 are the most rapid growers, those with broad or with nar- 

 row rings ? Do you notice any difference in the texture 

 of the wood in rapid and in slow growing trees ? Which 

 makes the better timber as a general thing, and why ? 



310. Vertical section through a black oak (from PINCHOT, U.S. Dept. of Agr.). 



236. Heartwood and Sapwood. Notice that in some 

 of your older specimens (cedar, black walnut, barberry, 

 black locust, chestnut, oak, Osage orange, show the differ- 

 ence distinctly) the central part is different in color and 

 texture from the rest. This is because the sap gradually 

 abandons the center (Sec. 224) to feed the outer layers 

 where growth in dicotyls takes place ; hence, the outer part 

 of the stem usually consists of sapwood, which is soft and 

 worthless as timber, while the dead interior forms the 



