THE WALLFLOWER 



97 



Then the outermost layer of the pericycle begins to 

 divide with great regularity by tangential walls (see 

 Fig. 37, pd), so that a cambial layer is formed, which, 

 like the true cambium, extends all round the stem. 

 This is called cork cambium or phellogen. It produces 

 cells, both on its inner and outer side. Those cut off 

 towards the inside retain their living contents, and 

 have rather thick cellulose 

 walls. There are several 

 layers of these cells in the 

 Wallflower; together they 

 constitute the phelloderm. 

 The cells cut off on the out- 

 side of the phellogen have 

 their walls suberised and 

 their contents disappear. 

 These cells form the cork. 



The whole of this second- 

 ary tissue, including phel- 

 loderm, phellogen, and cork, 

 is called the periderm. 

 The word bark is applied 

 to everything outside the 



FIG. 37. Part of a transverse 

 section of the stem of Wall- 

 flower, to show the outermost 

 layer of the pericycle dividing 

 up to form the periderm. c, 

 cortical cells; e, endodermis; 

 pd, phellogen, arising by divi- 

 sion of pericyclic cells. Mag- 

 nified 460. (E. S.) 



phellogen, and so here it includes the corky layers, 

 together with the endodermis and the rest of the 

 cortex and the epidermis. The bark consists entirely 

 of dead tissues, for the corky walls allow of no 

 absorption of water or food -materials. It is, however, 

 of great importance as a protection to the living 

 tissues within. Almost all plants with second;! ry 

 growth in thickness form periderm and bark through 

 it he activity of a phellogen, which, however, does not 

 7 



