HE ART WOOD AND SAP WOOD. 23 



Pis. Vll-X.) It is true that two false rings may appear 

 iu one year, but they are generally so much thinner 

 than the rings on each side that it is not hard to detect 

 them. Very often they do not extend entirely around 

 the tree, as a true ring always does if the tree is sound. 

 Whenever the growth of the tree is interrupted and 

 begins again daring the same season, such a false ring 

 is formed. This happens when the foliage is destroyed 

 by caterpillars and grows again in the same season, or 

 when a very severe drought in early summer stops 

 growth for a time, and in similar cases. 



HEAKTWOOD AND SAPWOOD. 



An annual layer once formed does not change in size 

 or place during the healthy life of the tree, except that 

 it is covered in time by other younger layers. A nail 

 driven into a tree C feet from the ground will still be 

 at the same height after it is buried under 20 or oO or 

 100 layers of annual growth. But in most trees, like the 

 Oaks and Pines, the wood becomes darker in color and 

 harder after it has been in the tree for some years. The 

 openings of its cells become choked so that tlie sap can 

 no longer run through them. From living sapwood, in 

 which growth is going on, it becomes heartwood, which 

 is dead, because it has nothing to do with growth. (See 

 figs. 19, 21.) It is simply a strong framework which helps 

 to support the living parts of the tree. This is why hol- 

 low trees may flourish and bear fruit. Sapwood rots 

 more easily than heartwood, because it takes up water 

 readily and contains plant food, which decays very fast. 

 Xot all trees have heartwood, and in many the differ- 

 ence in color between it and the sapwoodis very slight. 

 Since water from the roots rises only iu the sapwood, it 



