STRUCTURE OF WOODY STEMS 



199 



ing the bark and wood, reaching part way or all the way to the 

 pith. 



The bark, characteristic of woody plants, is originated by the 

 cork cambium which forms as an inner layer of the epidermis 

 or in the cortex beneath. (Fig. 178.) As the branch increases 



FIG. 178. Diagrammatic drawing of cross, radial, and tangental sections 

 of three-year old Basswoodstem. a, lenticels; b, epidermis; c, cork and cork 

 cambium ; d, bast fibers and conductive phloem, forming wedge-shaped patches 

 with points out and separated by the expanded ends of the medullary rays; 

 e, cambium; /, cortex; g, medullary rays; h, pith; i, pitted conductive tubes 

 of xylem; ,;', wood fibers of xylem; k, spiral conductive tubes of xylem. (Illus- 

 tration planned in general by permission after figure 57 in Nature and Devel- 

 opment of Plants," by C. C. Curtis, published by Henry Holt and Company.) 



in diameter, the epidermis seldom grows in proportion, but usu- 

 ally dies and sloughs off, and its protective function is assumed 



