THE SEQUOIA 
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pene A 
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DIAGRAM OF THE STRUCTURE OF THE STEM OF AN EXOGENOUS TREE OF THREE 
YEAkS’ GROWTH 
Modified from a cut in “* Encyclopedia Britannica” 
Pp, pith; cc, Cambium cells; b, bark; 1, 2, 3, growth of wood during first, second and third year. 
2 inches in diameter, measured inside the bark, which in places 
is nearly a foot in thickness. 
The Big Tree, like most trees of temperate climates, is ex- 
ogenous, as is indicated by the concentric circles of wood beauti- 
fully shown in this specimen. 
A transverse section of the stem of any exogenous tree of one 
year’s growth consists of three distinct areas or zones. In the 
center is the pith, around this a ring of wood, and surrounding the 
whole the bark. Each of these layers consists of cells which are 
variously modified to carry on their respective functions. Uniting 
the bark and the wood are delicate thin-walled cells, filled with 
protoplasm and nutrient matter, which constitute the zone of 
growth of the tree. The innermost cells of this Cambium layer, 
as it is called, form the new wood, while the outermost renew the 
bark. The oldest wood, then, is that nearest the pith, while the 
oldest bark forms the exterior of the trunk. During the spring, 
when the sap is running, the multiplication of Cambium cells is 
very rapid, and consequently more wood and bark are laid down 
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