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- 
Fic. 178. — Diagrammatic drawing of a cross section of an apple twig 
after completing two seasons growth. e, epidermis sloughing off; k, cork; 
h, cork cambium; i, inner cortex; 7, the phloem formed the first season; 
p, phloem formed the second season; c, cambium; z, 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 
