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 

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



FIG. 178. Diagrammatic drawing -of a cross section of an apple twig 

 after completing two seasons growth, e, epidermis sloughing off; fc, cork; 

 h, cork cambium; i, inner cortex; n, the phloem formed the first season; 

 p, phloem formed the second season; c, cambium; x, xylem formed the 

 first season; y, xylem formed the second season. 



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

 the cork formed beneath and gradually thickened as the stem 

 grows older. In some cases the cork cambium produces cortex 

 cells on its inner side as well as cork on its outer side, in which 

 case the cortex is increased in thickness. Since cork is imper- 

 vious to water, the tissues on its outside, having their water 

 supply cut off, soon die and with the epidermis and cork form 



