TREE TRUNK 67 



All through the summer the sap passes back and forth be- 

 tween the roots and leaves. In going toward the leaves it 

 passes through the pores, or open tubes in the wood. Did you 

 see these pores in the specimen of wood that you studied? 

 Wood is mostly bundles of thick-walled tubes, but it often 

 takes sharp eyes to see their openings. 



Does sap go up through all the wood ? Find out by putting 

 the cut end of an apple twig in water colored with ink. Let it 

 stand for a day or two. Then split it open lengthwise and see 

 in what part of the wood the ink has risen. 



In going from the leaves toward the roots sap passes 

 through the inner parts of the bark. 



How a Tree repairs its Wounds. Look at the place where 

 a limb has been sawed from a tree. Which part of the wound 

 is the bark covering the more rapidly, the upper or the lower ? 



You may have seen bark trying to cover the stump of a limb 

 that was broken off at some distance from the trunk. Often 

 the wood decays before the bark can cover it, and then the 

 whole inside of the trunk is in danger of becoming rotten. In 

 pruning a tree, always saw off a limb with a smooth cut close 

 to the trunk. 



Have you seen a swelling on a tree where a tight band has 

 been put around it ? Is the swelling above or below the band ? 

 The sap in the bark can not readily flow past the band. Thus 

 the wood at the band holds back some of the food belonging 

 to the other parts of the tree, a.nd so that place grows more 

 rapidly than the rest of the trunk. 



Sap and Heart Wood. After a few years the tubes in the 

 wood of a tree become filled up so that the older wood is 

 firmer and stronger than the young wood. In a large tree the 

 center of the trunk is darker in color, and harder than the 

 outer part. The center is called heart wood, and makes better 

 lumber than the outer or sap wood. 



Grain Marks on Wood. In some kinds of wood the marks 

 made by each year's growth show more plainly than they do 



