

The Structure of a Tree 15 



pith of a tree is composed of cells of this kind, and in a 

 freshly cut stump or log strands of these cells, called 

 medullary rays, or pith rays, may be seen running out 

 from the center to the bark. Cells of this kind may also 

 be scattered through the wood ; or they may lie in 

 .groups or run in chains, crosswise or up and down, in 

 the wood, making connections with each other and 

 with the living portions of the bark. These living cells 

 are found in the wood of the roots as well as in the trunks 

 and branches of trees. They make up a considerable 

 part of many woods, sometimes as much as a quarter 

 of the whole, and along with the tracheae are the weakest 

 elements in the wood. If the bark were hewed from a 

 pine, it is estimated that the ends of 15,000 fine rays 

 would be exposed on each square inch of surface. In 

 oaks some of the rays are an inch, and in some species of 

 oak even 4 or 5 inches, in height, and may be seen as 

 light-colored bands running through the wood (page 103) ; 

 but for each large ray that you see in a piece of oak 

 there are perhaps one hundred too small to be seen with- 

 out a lens. 



These four elements are found arranged in various 

 ways and in different proportions in all our hard woods 

 (page 109). The woods of pines, cedars, firs, and other 

 evergreens are composed of tracheids and parenchyma 

 cells. They lack the large open vessels and the dense 

 bundles of wood fibers and are therefore very even in 

 texture. 



How a tree grows in height. In the tip of a tender 

 root or shoot each cell is growing; it increases in size 

 by taking in food and out of this food building up more 



