THE STRUCTURE AND WORK OF THE STEM 103 



Section across a young twig of box 

 elder, showing the four stem regions : 

 e, epidermis, represented by the 

 heavy bounding line ; c, cortex ; w, 

 wood; p, pith. (From Coulter, 

 Plant Relations.) 



distinct regions. The center is occupied by the spongy, soft pith; 



surrounding this is found the rather tough wood, while the outer- 

 most area is called cortex or bark. 



More careful study of the bark 



reveals the presence of three 



layers an outer layer, a middle 



green layer, and an inner fibrous 



layer, the latter usually brown in 



color. This layer is made up 



largely of tough fiberlike cells 



known as bast fibers. The most 



important parts of this inner 



bark, so far as the plant is con- 

 cerned, are many tubelike struc- 

 tures known as sieve tubes. 



These are long rows of living 



cells, having perforated sievelike 



ends. Through these cells food 



materials pass downward from the upper part of the plant, where 



they are manufactured. 



In the wood will be noticed 

 (see Figure) a number of lines 

 radiating outward from the 

 pith toward the cortex. These 

 are the so-called medullary 

 rays, thin plates of pith which 

 separate the wood into a num- 

 ber of wedge-shaped masses. 

 These masses of wood are 

 composed of many elongated 

 cells, which, placed end to 

 end, form thousands of little 

 tubes connecting the leaves 

 with the roots. In addition 

 to these are many thick-walled 

 cells, which give strength to 

 the mass of wood. In sec- 

 tions of wood which have 



Section across a twig of box elder three 

 years old, showing three annual growth 

 rings in the vascular cylinder. The ra- 

 diating lines (m), which cross the wood 

 (w), represent the pith rays, the prin- 

 cipal ones extending from the pith to the 

 cortex (c). (From Coulter, Plant Rela- 

 tions.) 



