222 THE SECOND BOOK OF BOTANY. 



The changes produced by age upon the woody 

 system of the stem, besides its annual addition of a 

 woody layer, constituted as above described, are, 

 first, loss of color of the cells of the pith, which at 

 last dry up, and lose all vitality ; the thickening of 

 the wood-fibres by internal deposit, while, at the 

 same time, they take on a dark color, and become 

 duramen, or heart-wood. 



Can you see any reason why the yearly layers of 

 wood should form rings so distinct that they can be 

 counted, and the age of the tree determined ? It is 

 because the wood formed in the first part of the year, 

 which is, of course, placed next the old ring, is more 

 porous, and often has a larger number of ducts with 

 large mouths than the wood formed later in the sea- 

 son. This is shown in Fig. 394, where the annual 



FIG. 394. 



layers are marked off and numbered, and you see 

 that the inner portion of the layer is more porous 

 than the outer. 



Now, the new layer of wood is moulded exactly 

 upon the last year's layer, so that the bundles are 

 separated, as before, by the medullary rays, which 



