THE CHAEACTERISTICS OF TREES. 21 



first very thin, but subject to great alterations with age, 

 owing to the distention through the increasing diameter 

 of the stem, as well as the formation of new layers of 

 liber or inner bark. This annual addition from within 

 pushes outward the older bark, often causing it to crack 

 open, forming deep fissures in the outer surface, or to 

 fall off in scales. 



The stem of the tree is composed of wood in different 

 conditions. The term alburnum is aj^plied to the new 

 or sap-wood, through which the crude sap absorbed by 

 the roots passes upward to the leaves where it is assimi- 

 lated. In returning, it is distributed over the entire sur- 

 face of the tree, forming new layers of wood and bark. 

 Some authors have applied this name to the half-formed 

 vegetable matter, lying between the bark and the wood 

 during the growing season, but that is now called by 

 vegetable physiologists the "cambium layer," to dis- 

 tinguish alburnous or fully formed young wood. This 

 alburnum or sap-wood, sooner or later, is mostly 

 changed into heart-wood, assuming in most kinds of trees 

 a dark color as seen in the red cedar, black walnut, 

 beech, and oak, and although it is in fact dead wood, 

 decay is prevented through its protection from the air, 

 by the surrounding layers of alburnum. 



Some kinds of trees have very thin layers of alburnum, 

 especially those of slow growth, while the stems of 

 others appear to be all sap-wood, as seen in the white 

 pine, tulip tree, and white-heart hickory, but this is 

 more in appearance than reality, the difference in color 

 between the old and new wood being but slight. The 

 change from soft-wood to heart-wood is not sudden, but 

 proceeds slowly, the cell walls gradually becoming thicker 

 and more rigid with age, and the difference in color is 

 due mainly to chemical changes. The alburnum or out- 

 side layers decay when exposed to the air far more rapid- 

 ly than the heart-wood ; consequently it is less valuable 



