104 THE OAK 
I now proceed to some further details, which could 
only be rendered intelligible in the light of the pre- 
ceding preliminary remarks. 
After the cambium ring is once formed the daughter- 
cells cut off on the inside of the cambium always 
become transformed into one or more of the following 
elements :— 
(1) Some cambium cells which lie on the radial con- 
tinuation of a medullary ray undergo a few horizontal 
divisions across the long axis, and then simply pass 
over as constituents of a medullary ray; as the cam- 
bium ring moves outwards, in consequence of the re- 
peated formation of thickening rings, the periphery of 
the cylinder of course increases, and this allows of more 
space tangentially. One consequence of this is the 
occasional and gradual widening of the medullary ray in 
process of lengthening: this takes place to a small 
extent only. Another consequence of the increased 
space is the occasional interpolation of new medullary 
rays. Radial rows of cambial cells at points which lie 
between the planes of two gradually diverging medul- 
lary rays suddenly commence to form new medullary 
rays. Hence, as the wood mass increases in radial 
thickness, more and more of these interpolated medul- 
lary rays appear, cutting up the wood proper into 
partial sections. In succeeding years the cambium 
keeps adding to the length of these rays, as it does to 
that of the older rays, and again forms new ones between 
as space increases. In the same ring about thirteen 
