THE WALLFLOWER 91 



gradually become converted into vessels, parenchyma, 

 or fibres of the wood. In the same way the cells cut 

 off on the outer side of the cambium divide once or 

 twice, and then become sieve-tubes, companion-cells, 

 or parenchyma of the bast. The amount of wood 

 formed on the inside is much greater than the amount 

 of bast formed on the outside of the cambium. Now, 

 this production of new xylem and phloem only adds 

 to the size of each individual vascular bundle. In (in 

 old Wallflower stem, however, we find that the vascular 

 bundles are no longer isolated, but there is a thick 

 continuous zone of wood, surrounded by a narrower 

 zone of bast. How is this brought about ? It depends 

 on the fact that the cambium does not remain isolated 

 within each bundle, but becomes united to form a 

 complete ring all round the central cylinder of the 

 stem. 



When the stem is quite young, we find the vascular 

 bundles separated from one another by radial bands of 

 conjunctive parenchyma the primary medullary rays 

 (see p. 48, Fig. 20, if). There is already an active 

 cambial layer between the xylem and phloem of each 

 bundle, and soon the infection of division spreads to 

 the neighbouring cells of the ray parenchyma, which 

 also begin to divide up by tangential walls. Thus a 

 layer of cambium is formed across each primary ray, 

 joining the cambium of the bundles on either side, and 

 so the whole central cylinder comes to be encircled by 

 a zone of actively dividing cells, producing new wood 

 internally and new bast externally. The cambium 

 within the vascular bundles, derived from the original 



