TYPES OF STEMS. 



381 



are killed back more or less. Next season's shoot is a branch 

 from some axillary bud. Such growth is termed indefinite. 



743. Structure of woody stems. If we make a cross-section of a woody 

 twig three general regions are presented to view. On the outside is the 

 rather soft,often greenish " bark," 

 so called, made up of sieve - 

 tubes, ordinary parenchyma 

 cells, and in many cases long 

 fibrous cells composing the "fi- 

 brous bark." From a growing 

 layer in this region, termed the 

 phellogen, the true corky bark 

 of the older trunk is formed. 



Next within the bark we find 

 the so-called "woody" portion 

 of the twig. This is strong and 

 resistant to both breaking and 

 cutting. The microscope shows 

 it to be composed of the ordi- 

 nary already known woody ele- 

 ments,* wood-fibers, for 

 strengthening purposes, pitted 

 and spiral vessels as conducting 

 tissue ; and intermixed with these 

 some living parenchyma cells. 

 A cross-section of the stem also 

 shows narrow radial lines through 

 the wood. These are pith-rays, 

 composed of vertical plates of 

 living parenchyma cells. These 

 cells, unlike the others in the 

 wood, are elongated radially, 

 not vertically. The height of the 

 pith-rays as well as their thick- 

 ness varies with the species studied. In the older trunk only the outer por- 

 tion, a few inches in thickness, remains light-colored and fresh, and is called 

 sap-wood. The inner wood is usually darker and harder, and is termed 

 heart-wood. Living parenchyma cells, in general, are present only in the 

 sap-wood, and in this almost solely the ascent of sap occurs. Dyestuffs 

 and other substances are frequently deposited in the walls of the heart-wood. 



The third region occupying the center of the twig is the pith. This 



* Chapter V, and Organization of Tissues in Chapter XXXVIII. 



Fig. 432. 



Three-year-old twig of the American ash, 

 with sections of each year's growth showing 

 annual rings. 



