100 THE 



has also been formed between these outer protective 

 layers and the cambium. 



We have now to obtain some ideas as to these curi- 

 ous processes of increase in thickness of the stems and 

 branches. 



The first thing to insure this is to understand the 

 constitution and behavior 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 pri- 

 mary vascular bundle is separated from the phloem of 

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

 and 12) ; we also saw that the bundles are arranged in a 

 closed ring round the pith, and are in their turn sur- 

 rounded by the primary cortex, each being separated 

 laterally from its neighbors 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 pri- 

 mary medullary rays, therefore, are merely spokes, as it 

 were, joining the pith and cortex ; and if we could re- 

 move the whole of the vascular bundles and epidermis 

 from the young stem we should have left a solid cylin- 

 der of cell (pith) in the center, 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 



