THE WALLFLOWER 9D 



groups. The innermost of these pericyclic cells, i.e. 

 those lying next the protoxylem, now undergo regular 

 tangential divisions, and so form the continuation of 

 the cambium round the ends of the xylem-plate. It 

 is a constant rule in plants of the class to which the 

 Wallflower belongs, as well as in plants of the Fir 

 kind, which belong to a different Class, that the forma- 

 tion of the cambium in the root begins by division of 

 the conjunctive cells inside the phloem-groups, and is 

 completed by division of the pericyclic cells outside 

 the xylem-groups. Thus the cambium (as seen in 

 transverse section) cannot at first form a perfect circle, 

 for it has a depression opposite each phloem bundle ; 

 but just at these places the cambium starts its work 

 earliest, and is for a time most active, so that the 

 inequalities soon become equalised, and we have a 

 circular zone of actively dividing cells just as in the 

 stem. When once started, there is nothing peculiar 

 about the cambium of the root; it forms internal 

 wood and external bast just as it does in the stem, 

 but as in this root there is no pith, it is evident there 

 can be no principal medullary rays. One point 

 deserves special attention : the primary xylem of the 

 root develops, as we have seen, from without inwards ; 

 the secondary xylem is formed by the cambium from 

 within outwards, so that in roots we have a sharp 

 distinction between centripetal primary and centrifugal 

 secondary wood, whereas in the stem all the wood 

 arises centrifugally. However old the Wallflower root 

 may be, we can always tell its transverse section from 

 that of the stem, for in the middle, instead of the pith, 

 we find the little diarch xylem-plate unaltered. Tn 



