92 THE OAK 



tangential walls. The result of this activity is the 

 development of a cambium layer, as it is called, immedi- 

 ately inside the five phloem groups of the axis-cylinder, 

 and this layer becomes continuous all round the axis- 

 cylinder, but is so arranged that it runs outside the 

 primary xylem groups and inside the primary phloem 

 groups (fig. 24, cam). This cambium layer is a hollow 

 cylindrical layer of thin- walled cells, full of protoplasm, 

 and somewhat longer than they are broad or deep, and 

 these cells have the peculiarity of dividing very rapidly, 

 especially by tangential walls, so that cell multiplication 

 goes on very rapidly, and the layer would soon become 

 very thick if no other changes occurred. As the new 

 cells are formed, however, those on the outer side of the 

 cylinder i.e. those nearest the phloem become for the 

 most part converted into sieve-tubes and cells of the 

 phloem ; while the much more numerous cells formed 

 on the inner side i.e. nearest the centre of the axis- 

 cylinder are chiefly converted into vessels and cells of 

 the xylem. This xylem and phloem developed by the 

 cambium are termed secondary xylem and secondary 

 phloem respectively, and it will be noticed that whereas 

 the secondary phloem is deposited radially on the inner 

 side of the primary phloem, the secondary xylem is 

 placed between the primary xylem groups, and not radially 

 outside them (fig. 24, se.x and se.pK). Moreover, the 

 youngest vessels are now nearest the cambium, whence 

 the order of development has become the converse of that 

 of the primary xylem ; there are also no spiral vessels 



