4 6 



ANATOMY OF THE GYMNOSPERMS 



tw. 



in a transverse section it approximates more nearly to the primi- 

 tive form and thereby exhibits a hexagonal outline as directly 

 derived from the circular by mutual compression of contiguous 

 elements. As the woody elements arise by division, their tend- 

 ency at first is to assume the spherical form ; hence they split 

 away from one another. But such separation is never fully com- 

 pleted ; in fact, it is never developed to any marked extent in 

 woody tissues and it remains localized. The separation is usu- 

 ally confined to the angles of the tracheids, and it results in the 



formation of intercellular spaces, 

 which therefore originate scJii- 

 zogcnously (fig. 3). In the subse- 

 quent modification of the wall by 

 secondary growth these spaces, 

 when not too large, are com- 

 monly obliterated by infiltration. 

 The presence of such intercellu- 

 lar spaces always emphasizes the 

 fact that what appears as a single 

 membrane is in reality a double 

 s-w. wall, one half of which belongs 



FIG. 3. PSEUDOTSUGA DoucLAsii. toeach of the adjacent cavities. 



Transverse section showing the struc- j n thg secondary growth of the 

 ture of the cell wall: p.w., primary 



wall; j.w., secondary wall ; t.w. t ter- wall new layers are laid down 



tiary wall; *j., intercellular space. on opposite Sides of this primary 



wall (fig. 3), and in their devel- 

 opment they give the dominant features of thickness, color, and 

 hardness (fig. 3, s.zv.). When brought into such relations the pri- 

 mary wall, apparently lying between two cells, is often designated 

 the intercellular substance. The secondary wall not only exhibits 

 very variable development in thickness according to the condi- 

 tions of growth, as between the spring and the summer woods, 

 but its growth is also more or less localized, and there thus arise 

 such structural features as have already been described in the 

 spiral tracheids, or such as will be discussed more in detail in 

 a subsequent chapter under the title of "Bordered Pits." Upon 



