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 phloém of 
the same bundle by a thin strand of cambium (figs. 9 and 
12); wealso 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. Hach spoke is very thin from side 
