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 phloem. 



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 i.e. nearest the pith, and therefore abutting on 

 the xylein 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 phloem) 

 become new phloem 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 phloem and cortex, adding new phloem 

 elements (but in far less proportion) to the inside of the 



