156 



All Food Is Made by Gree?i Pimm unit hi 



constantly growing in diameter (width) 

 throughout its entire length. 



The growth in thickness is the result 

 of the activity of the microscopic layer 

 of cambium between the phloem and 

 xylem. At the end of the w inter when 

 the ground thaws, or at the beginning of 

 the rainy season where there is no frost, 

 water again enters the roots, and sap be- 

 gins to flow upward. I'hcn the cells of 

 the cambium divide actively and form 

 thick-walled xylem tubes and fibers to- 

 ward the inside. At the same time they 

 produce more of the sieve tubes and 

 fibers toward the outside. See Fig. 179, 

 page 1 54. The xylem tubes formed in the 

 spring of the year are wider cells than 



Fig. 183 (above) Cross section of a 30-year-old 

 part of a tree. How can its age be told? Notice 

 the dark center portion, called heartwood, and 

 the lighter outer portions, called sapwood. 

 Notice, too, that the thickness of the annual 

 ririgs varies. Can you think of some reason for 

 this variation? 



Fig. 182 (left) California redwoods. So?ne of 

 these were already tall trees when Columbzis dis- 

 covered Afnerica. How do we know this? (u. s. 



DEPARTMENT OF INTERIOR) 



those formed in the late summer. Even 

 to the naked eye the spring growth looks 

 quite different from the summer growth. 

 The narrow cells formed late in the sea- 

 son show as a darker "ring" (cylinder). 

 These double rings are called aimiial 

 ri?igs. By counting them one can learn 

 how old that part of the tree is. There 

 are, of course, more rings at the base 

 than near the top of a tree. No rings 

 appear in the phloem region because 

 there is no difference in size of phloem 

 cells that are formed in spring and sum- 

 mer. The cork of most trees likewise 

 has no annual rings but in birch bark 

 cork the cells made in spring are thinner 

 walled than those made in summer; that 



