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 phloém 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 phloém 
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 phloém—become for the 
most part converted into sieve-tubes and cells of the 
phloém; 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 phloém developed by the 
cambium are termed secondary xylem and secondary 
phloém respectively, and it will be noticed that whereas 
the secondary phloém is deposited radially on the inner 
side of the primary phloém, the secondary xylem is 
placed between the primary xylem groups, and not radially 
outside them (fig. 24, sew and se.ph). 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 
