i66 



STEMS 



Fig. 56. — Section of a one- 

 year-old twig of box elder. 



ture as the twig has now. How have all these changes 

 come to pass? If we are to understand them, we had 

 better study the twig first and then 

 see what happens to it as it grows 

 older. 



A one-year-old twig of any com- 

 mon tree shows four distinct tissue 

 regions. (See Figure j6.) Outer- 

 most there is the epidermis.; it is a 

 thin skin that often may be easily 

 stripped off. Under the epidermis 

 Hes soft tissue usually containing 

 chlorophyll; this is the cortex. Next comes a zone of 

 hard tissue; this is the vascular cylinder; it is chiefly 

 composed of wood. Finally, in the center, there is the 

 pith, a white, spongy tissue most of which soon dies. 



If we examine a three- 

 or four-year-old twig of 

 the same tree, we find 

 that a great change has 

 occurred. (See Figure 57.) 

 The tender epidermis has 

 disappeared and a new 

 and darker protective 

 covering has been devel- 

 oped by the outer part of 

 the cortex. This new 

 covering is called bark. 

 The cortex has lost much 

 if not all of its chlorophyll ; it is ceasing to be an organ of 

 photosynthesis ; the bark does not let light or gases through 

 as the epidermis did. The wood cylinder has become 



57. — Section of a three-year-old twig 

 of box elder. 



