ELEMENTS PRODUCED EV THE CAMBIUM I25 



of a branch become buried by the wood added to the stem 

 year by year : on cutting a longitudinal board as indicated at 

 C, the buried part of the branch is cut almost transversely, and 

 appears as an oval knot {k). 



(c) Elements produced by the cambium : medullary rays. — As 

 the cambium lies between the wood and bast, it is obvious that 

 'Cas, primary first-formed wood and bast of the vascular bundles 



Fig. S%.—A, Stem of a tree six years old with branch b ; B, longitudinal section of the 

 same, showing all annu.il rings of stems except the first continued in branch; D, longi- 

 tudinal board cut from A ; k knot (transverse section of branch i>). 



must be gradually pushed further apart by the secondary wood 

 and bast produced by the cambium, so that in old stems the 

 primary wood is found surrounding the pith in the centre, 

 while the primary bast is met with near the outside (C, Fig. 60). 

 The elements forming the secondary wood are similar to those 



