54 A TEXT-BOOK OF BOTANY 
of concentric rings (Fig. 54). Ordinarily one such layer is 
added each year, and hence the layers are called annual 
rings. The age of a tree is usually estimated by counting 
these rings, but occasion- 
ally more than one ring 
may be added during a 
single year. The new 
layers added to the bast 
are not persistent; but 
the wood accumulates 
year after year, until in 
an ordinary tree the 
stem is a great mass of 
wood covered with thin 
layers of bast and cor- 
tex. It is this mass of 
Fic. 54.—Cross-section of a branch of box a : 
elder three years old, showing three an- W ood that supplies our 
nual rings in the vascular cylinder; the |umber. 
radiating lines (m) which cross the vascu- ' ‘ 
lar ring (w) represent the pith rays, the This annual increase 
principal ones extending from pith to cor- jy diameter enables the 
oe tree to put out an in- 
creased number of branches, and hence leaves, each suc- 
ceeding year, so that its capacity for leaf work becomes 
greater year after year. A reason for this is that since the 
wood is conducting water to the leaves, for food manufac- 
ture, the new layers enable it to conduct more water, and 
more leaves can be supplied. 
When a stem increases in diameter it is very seldom 
that the epidermis grows in proportion. Hence it is usu- 
ally sloughed off and a new protective covering is de- 
veloped by the cortex. Either the outermost layer of the 
cortex or some deeper one becomes a cambium, which 
means that it is able to form new cells. This cambium is 
called the cork cambium, since it forms at its outer surface 
layer after layer of cork cells, which are peculiarly resistant 
