102 TUB 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 realize 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, 

 lose their cambial nature and the xylem become con- 

 verted into elements of the wood ; while a smaller pro- 

 portion 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 cell 

 travels outwards ; ever receding radially farther 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 

 phloem. Each winter it pauses in this process, and 



