THE LIVING PLANT 



25 



The outer portion of each bundle also consists of modified 



cells and is known as the phloem, or true bark. Between the 



phloem and the xylem is a thin tissue of cells known as 



the cambium. It will 



also be seen that the 



cambium extends in 



a circle through the 



pith. The cambium 



cells are the growing 



portions and build up 



the other parts of the 



stem. It is through 



the vascular bundles 



that solutions travel 



upward and down- V&8&W- or* 



ward through the 



tree. The solutions 



travel upward almost 



entirely through the 



xylem and downward 



through the phloem. Specially modified cells, called tracheal 



tubes, occur in the xylem for the passage of solutions upward. 



Other modified cells, known as sieve-tubes, are present in 



the phloem for the passage of the solutions downward. 



Both of these types of cells carry solutions after the cells are 



dead. They are illustrated in Fig. 12. These are not the 



only types of cells in the vascular bundles. It should be 



noticed also that, although most of the movement of solutions 



through the stem is upward and downward, some movement 



takes place in other directions. 



As the stem becomes older, the vascular bundles press the 

 pith between them into thin plates called medullary rays. 

 In old stems the xylem forms a continuous ring of wood and 

 the phloem a continuous ring of bark with the cambium 

 between the wood and bark. Each growing season the cam- 



FIG. 12. A section of same stem as in Fig. 11, older, 

 with vascular bundles closer together and pith 

 between compressed into medullary rays. 



