[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 263 



If blocks of older and larger woody stems are examined, it is evident 

 that the wood cylinder has increased in diameter, that the bark is 

 somewhat thicker than in younger stems, and that the diameter of the 

 pith has remained unchanged. Evidently growth in diameter of stems 

 consists of the formation of new xylem and, to a less extent, of new bark 

 tissues. 



When the older wood in the central part of the stem becomes darker in 

 color it is referred to as heartwood. The younger, lighter-colored wood is 

 called sapwood. Almost all the older wood is dead; and when fungi that 

 can digest and oxidize the substances in the cell walls gain entrance into 

 the center of the stem through wounds, they may destroy all but a thin 

 shell of sapwood adjacent to the bark. Such hollow trees, however, may 

 continue to live and grow for many years (Figs. 94 and 101 ). 



1 ■ 



Fig. 94. Photograph of a hollow log from a sycamore tree which lived and 

 grew for many years after the heartwood had been destroyed by fungi. See also 

 Fig. 101. 



It is evident from the differences in color and texture that the bark is 

 composed of several kinds of tissues and that the cells of the wood 

 cylinder are not all alike. The annual growth rings (ends of annual 

 cylinders of xylem) may usuallv be counted in the wood cylinder, 

 though sometimes a hand lens or even a microscope is necessary to 

 distinguish them clearly. A careful examination of the ends of the 



