102 THE OAK 
lary rays, and so forms a complete thin cylinder, con- 
centric to the pith—from which it is separated by the 
breadth of the xylem—and the cortex, from which it 
is separated by the breadth of the phloém. 
The cells of this cambium cylinder go on dividing 
continuously during the whole summer, until the 
cylinder is, say, ten times as thick as it was before. 
Now suppose it to rest during the winter and go on 
again next season, and so on during each successive 
period of growth. Obviously this would realise one 
fact in the process we are considering—namely, that the 
stem would grow in thickness year by year, its diameter 
being increased by twice the thickness of the added 
cylinder. 
But to make the above supposition accord with the 
facts, we must further picture to ourselves that when the 
thickening cylinder has attained a certain thickness, a 
large proportion of those of its cells which lie on the 
inside—1z.e, nearest the pith, and therefore abutting on 
the xylem—-lose their cambial nature and become con- 
verted into elements of the wood ; while a smaller propor- 
tion of those on the outer side (beneath the phloém) 
become new phloém elements. In this way it will be 
seen that the thin cylinder of active cambium cells 
travels outwards; ever receding radially further from the 
pith, and leaving xylem between itself and the primary 
vascular bundles next the pith, and ever driving outwards 
the primary phloém and cortex, adding new phloém 
elements (but in far less proportion) to the inside of the 
