THE SPRUCE FIR 259 



secondary bast on its exterior. As it is the layer of 

 cells next the phloem which divides to form the 

 cambium, a band of unaltered parenchyma is always 

 left between the primary and secondary wood (see 

 Figs. 104, 105, and especially 10G). 



For some time the cambial divisioDS are limited to 

 the part immediately within the phloem groups (see 

 Fig. 104). Eventually, however, the division spreads 

 to the pericyclic cells lying outside the two resin 

 canals, which border on the protoxylem groups. In 

 this way the cambium is completed round the ends of 

 the xylem-plate, and henceforward it forms a continu- 

 ous ring. Fig. 106 represents the stage when the 

 cambial divisions have already extended to the cells 

 beyond the resin canal. No secondary tissue has as 

 yet been formed in this position, although at the 

 sides of the xylem-plate a considerable amount of 

 secondary wood and bast has already been developed, 

 so that the primary phloem is crushed and obliterated. 

 A later stage, in which the secondary tissues already 

 form a complete ring, is shown in the diagrammatic 

 figure 105. The structure of the secondary tissues 

 needs no special description. It is the same as in the 

 stem. We may, however, notice that in the first few 

 layers of the secondary wood the pits are formed on 

 all surfaces of the tracheides indiscriminately (see Fig. 

 106, I). In the later-formed layers they are limited 

 to the radial walls (see, however, p. 245). 



As soon as the cambium has begun its activity, the 

 most external layer of the pericycle also becomes the 

 seat of new formations. The cells of this layer divide 



