THE SPRUCE FIR 257 



The first thing that strikes us as peculiar in the 

 structure of the cylinder is the great width of the 

 pericycle, which forms a broad zone from six to nine 

 cells in thickness. It is thickest opposite the middle 

 of the two phloem groups (see Fig. 104, ^>.c). Most 

 of the pericyclic cells contain a quantity of starch, 

 which is ready for use in the formation of the cell- 

 walls of the new tissues as they develop. Within 

 the pericycle comes the vascular tissue, which, as 

 seen in transverse section, occupies a somewhat 

 elliptical area (see Pig. 104). The two protoxylem 

 groups lie at the ends of the long axis of the ellipse, 

 and therefore at the points where the pericycle is 

 narrowest (Fig. 104, px). This first-formed xylem 

 consists of spiral tracheides. The tissue lying between 

 the two protoxylem groups slowly becomes converted 

 into wood. The differentiation advances from either 

 end towards the middle of the cylinder, until ultimately 

 a continuous plate of primary wood is formed, extend- 

 ing across the middle of the cylinder (see Figs. 104 

 and 105). The primary wood is now complete. Its 

 development goes on very gradually, so that the 

 secondary growth has usually begun before the 

 primary structure is finished. We will, 'however, 

 consider the two stages separately. All the middle 

 part of the primary plate of wood consists of pitted 

 tracheides. Just outside each protoxylem group a 

 resin canal is formed in the pericycle (see Figs. 104, 

 105, and 106). 



The primary phloem forms two bands, one on each 

 side of the xylem-plate, from which they are separated 

 17 



