PULP AND PAPER 



Libriform fibers are the main constituent 

 of pulp. There are many wide, short pits on 

 the cell wall. The tracheides are verj^ broad 

 and have wide simple pits. The border of 

 these is difficult to see. There are invariably 

 many parenchyma elements in the pulp. 

 These are elongated, thin-walled and very 

 porous. 



Birch (Betula alba). The vessels are char- 

 acterized by bordered pits very close to- 

 gether. The border is not visible. The wood 

 tracheides show simple pits, likewise very 

 close together. The cell wall is only moder- 

 ately lignate. Parenchyma cells are very thin 

 and occur only sporadically. 



Poplar (Populus alha). Bordered pits with 

 hexagonal simple pits without a border and 

 large pits in crosswise rows. The vessels 

 have large, pronounced end-protrusions. The 

 pits of the fibers are not arranged in long 

 rows. Some fibers have no pits. The paren- 

 chyma elements are often elongated. 



(3) Soft Wood Pulp. In the soft woods 

 one must distinguish between the fiber tra- 

 cheides, the medulla tracheides and the 

 parenchyma elements. The fiber tracheides 

 are 4 or 5 sided thick-walled elements. On 



all walls. Semi-bordered pits are also found 

 locally, i.e., where the tracheides border 

 upon the parenchyma cells. Their walls are 

 thicker in places and in cross and tangential 

 sections these often look like tangential bars. 

 The medulla parenchyma has ordinary pits 

 on the walls adjoining other parench3^ma 

 cells. The walls adjoining the tracheides have 

 semi-bordered pits. 



In sulfite pulp the parenchyma cells also 

 have pronounced resin particles. These par- 

 ticles, which are easily identifiable, are im- 

 portant in distinguishing between sulfite and 

 sulfate pulp. 



The surface of the primarj^ wall of the 

 Spring tracheides is greatly wrinkled during 

 pulping of the wood and subsequent drying, 

 while cracks also occur in it. Moreover, 

 small fibers readily split off. This effect is 

 particularly easy to see in sulfite pulp. In 

 Autumn tracheides this crazing effect is far 

 less pronounced. 



Sulfate pulp is usually obtained from firs, 

 the bordered pits appearing as large, square 

 holes. Besides the above differences between 

 sulfite and sulfate pulps the following may 

 apply: 



two walls they have large round bordered 

 pits in rows. Besides these large bordered 

 pits, the Spring tracheides, which have a 

 wide lumen, show also small, ordinary pits 

 irregularly distributed over the wall. The 

 width of these tracheides is about 75 ju- The 

 Autumn tracheides have only bordered pits. 

 Their width is about 40 n. The medulla tra- 

 cheides have small, round bordered pits on 



Straw pulp. Straw pulp consists mainly of 

 bast fibers somewhat resembling flax fibers. 

 Unlike flax, however, they are always pres- 

 ent in paper almost undamaged. Further- 

 more, the X-shaped displacements are lack- 

 ing. In addition there arc large, round 

 parenchyma cells without any contents. To 

 a less extent annular, spiral and reticular 

 vessels are found. Of ]irimary importance to 



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