1 66 



STEMS 



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 56.} Outer- 

 most there is the epidermis ; it is a 

 thin skin that often may be easily 

 FIG. 56. Section of a one- stripped off. Under the epidermis 



year-old twig of box elder. Ues ^ 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 

 This new 



called bdrk. FIG. 57. Section of a three-year-old twig 



of box elder. 



the cortex. 



covering is 



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 



