ANATOMY OF STEMS 



223 



changes occur in the color, com- 

 position, and structure of the 

 various elements, resulting in the 

 conversion of sapwood into heart- 

 wood. As sapwood ripens into 

 heartwood the walls of any re- 

 maining living cells of the xylem 

 become increasingly lignified, 

 death of these cells soon follow- 

 ing. The water content of the 

 tissues is generally reduced and 

 such compounds as oils, resins, 

 gums, and tannins accumulate in 

 the cells or cell walls. The 

 darker coloration of the heart- 

 wood of most species as compared 

 to the sapwood is due to such 

 accumulations. 



In mature trees the heart- 

 wood becomes merely a central 

 supporting column surrounded 

 by a cylinder of sapwood which 

 varies in thickness from a few to 

 many annual la5^ers, depending 

 upon the species and the environ- 

 mental conditions under which 

 the tree is growing. In some 

 species (apple, elm) the heart- 

 wood remains virtually saturated 



Fig. 58. Tangential section of a 

 small portion of the wood of white 

 pine {Pinus strobus). The vertically 

 oriented elongated elements are tra- 

 cheids. Bordered pits show in sec- 

 tional view in the tracheid and mar- 

 ginal ray cells. Simple pits show in 

 sectional view in other ray cells. 

 Half-bordered pits show between 

 marginal ray cells and other ray cells. 



wood ray 



simple pit 



