THE STRUCTURE AND WORK OF THE STEM 103 



Section across tt young twig of box 

 elder, sbtnviug the four stem regions : 

 c, epidermis, represented by the 

 heavy bounding line ; c, cortex ; w, 

 wood ; p, pith. (From Coulter, 

 Plant Relatiotis.) 



distinct regions. The center is oeeupied 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 

 la>'ers — an outer layer, a middle 

 green layer, and an inner fibrous 

 la>'er, the latter usualh' 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 f:ir as the plant is con- 

 cerned, are manj^ tubelike struc- 

 tures known as sie\(' tubes. 

 These are long rows of living 

 cells, having perforated sievelike 

 ends. Through these colls food 

 materials pass downwai'd from the upper part of the plant, where 



they are manufactured. 

 ,C 



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 wcnlfic-shaped masses. 

 These masses of wood are 

 composed of nuin>' elongated 

 cells, which, placed end to 

 end, form thousands of little 

 tulles connecting the leaves 

 with the roots. In addition 

 to these are man>' thick-walled 

 ('(>lls, which give strength to 

 the mass of wood. In sec- 

 tions of wood which have 



Section across u twig of l)ox elder three 

 j'cars old, show-ing three annual growth 

 rings in the vascular c.\linder. The ra- 

 diating lines (m), which cross the wood 

 (w), represent the pith raj-s, the prin- 

 cipal ones extending from the pith to the 

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

 tions.) 



