100 THE OAK 



have also been formed between these outer protective 

 layers and the cambium. 



We have now to obtain some ideas as to these curious 

 processes of increase in thickness of the stems and 

 branches. 



The first thing to ensure this is to understand the 

 constitution and behaviour of the cambium cylinder, for 

 it is principally this tissue which brings about the 

 changes we have to study. 



We saw in Chapter IV. that the xylem of each 

 primary vascular bundle is separated from the phloem of 

 the same bundle by a thin strand of cambium (figs. 9 and 

 1 2) ; we also saw that the bundles are arranged in a closed 

 ring round the pith, and are in their turn surrounded by 

 the primary cortex, each being separated laterally from 

 its neighbours by a primary medullary ray. The next 

 point to bear in mind is that these medullary rays (like 

 the pith and cortex) are merely parts of the general cell- 

 tissue, or fundamental tissue, through which the vascular 

 bundles run upwards and downwards with a tangentially 

 sinuous course from the leaves. The primary medullary 

 rays, therefore, are merely spokes, as it were, joining the 

 pith and cortex ; and if we could remove the whole of 

 the vascular bundles and epidermis from the young stem 

 we should have left a solid cylinder of cells (pith) in the 

 centre, a hollow cylinder (cortex) concentric to this, and 

 a space between the two bridged over at numerous places 

 by cellular spokes (medullary rays) radiating from the 

 pith to the cortex. Each spoke is very thin from side 



