THE SPEUCE FIR 241 



side the layer of cork dries up, and is cast off. Thus 

 the irregularities of the surface are removed, and 

 the new surface formed by the cork is smooth and 

 cylindrical. Later on, however, new layers of 

 phellogen arise, by the division of deeper cortical 

 layers, and so a scaly bark is formed, such as we find 

 on the older stem or branches. The bark, as we 

 know, consists of the cork, together with all the dried - 

 up tissue outside it. 



We will now go on at once to describe the changes 

 which take place in the twig when it becomes a thick 

 branch or trunk. After what we have already learned 

 of the development of the Wallflower, it is quite easy 

 to understand this process, though the change from a 

 little twig, jJg- of an inch thick, to the stem of a great 

 Fir-tree a yard or more in diameter, is certainly a 

 remarkable one. 



We have seen that there is always a layer of 

 cambium between the wood and bast of each bundle, 

 and that by the divisions of the interfascicular tissue 

 the cambium becomes united into a complete ring. 

 When the cambial cells divide, they cut off cells on the 

 inner side, which after one or more further divisions 

 become elements of the wood. In like manner the 

 sells cut off from the cambium on its outer side, after 

 subdividing, become part of the phloem. Hence, just 

 as in the Wallflower, the oldest part of the wood is 

 always that nearest the pith, while the oldest part of 

 the bast is that nearest the cortex. The most recently 

 formed part of each is found near the middle of the 

 secondary zone, next the cambium. 



16 



