152 A TEXTBOOK OF GENERAL BOTANY 
separated, either by mechanical grinding or by boiling with 
chemicals. The loose fibrous material that is obtained in either 
of these ways is known as wood pulp. While wet, this pulp 
is combined with resin, clay, or other materials to give body 
and added strength, after which the pulp is pressed into paper. 




Fig. 149. Stages in the formation of cork of oleander (Nerium oleander) 
Upper left, epidermis and outer part of cortex before the formation of a phel- 
logen layer. Left center, the epidermal cells have elongated and divided to form 
a row of epidermal cells and below this the phellogen layer. Lower left, the 
phellogen layer has produced a single row of cork cells between the epidermis 
and the phellogen. Right, the uppermost row of cells is the epidermis, the next 
four rows are cork, while the sixth row from the top is phellogen. (x 250) 
Secondary changes in the bark. As the wood of the stem is 
enlarged by the cambial activity there is a tendency for the 
regions outside of the younger phloém to be stretched. This 
is partly compensated for by radial divisions of the parenchyma 
cells. When, however, there is a band of sclerenchyma, such 
as occurs in the pericycle of some stems (Fig. 113), the dead 

