THE TISSUES OF THE STEM 55 



and divide, acting as a cambium-cell. The cells thrown off from it 

 on the inner side take the characters of additional cortex ; those on 

 the outer side enlarge, their cell-contents are absorbed, and their 



FIG. 40. 



Diagrams illustrating successive steps in the formation of cork in the cells (2) 

 directly below the epidermis (i). A shows the first division. B shows the result of 

 repeated divisions, resulting in a radial row of cells. Of these the outer (a) are cork ; 

 the innermost (c) are phelloderm, which adds to the cortex, (b) represents the cork- 

 cambium. 



thin walls are changed to impervious cork (Fig. 41 : parts right and 

 left of the drawing). As the cells fit closely without intercellular 

 spaces, they form a complete protective covering. Partly owing to the 



FIG. 41. 



Transverse section of the stem of Elder, traversing a lenticel. e= epidermis. 

 p h = phellogen, or cork cambium. I = spongy cells filling the lenticel. pi = phellogen 

 of the lenticel. pd= phelloderm. ( x 90.) S. 



growth in bulk within, but chiefly because the cork cuts it off from 



the living tissues, the epidermis soon dries up, splits, and peels away. 



The covering of cork having its cells closely fitting together is not 



only impervious to liquids, but also to gases. Thus the living tissues 



