64 THE OAK. 



comes in of deciding whether a continuation occurs at a 

 higher or lower level. 



The cells of the cambium, seen in transverse section, 

 are rectangular in shape and arranged in regular radial 

 rows, owing to the regular tangential divisions (Fig. 12, 

 w, m). In longitudinal sections they are found to be 

 like the tracheids in shape and size, so that they stand 

 one behind the other at the same level. Regarding the 

 tangential series in rings, however, they are less regular, 

 because the tangential longitudinal divisions of two cells 

 side by side do not lie in the same tangential plane. 

 This regular radial arrangement would be found in the 

 xylem also, and is so to a certain extent, but it is dis- 

 turbed by the differences in diameter which the various 

 elements attain later. The fibers are most apt to pre- 

 serve the regularity, but in many cases growth in length, 

 and the intercalation of oblique septa, disturb it. 



In later years the length of the cambial cells in- 

 creases, and hence the length of the elements in the 

 wood. 



The phloem or bast of the individual bundle is sepa- 

 rated from its neighbors by large rays of parenchyma, 

 the cells of which agree with the secondary bast-paren- 

 chyma rays. As these pass into the cortex they widen, 

 as they do at the pith (Fig. 12). 



The oldest portion of the phloem that next the 

 cortex consists of a group of thick-walled bast fibers 

 with their lumina nearly obliterated; these are long, 

 spindle-shaped fibers much like the fibers of the wood. 



