THE STKUCTUKE OF WOOD. 



39 



but not over I3//' in diameter. A Jarge knot is sound, and over 1>4" 

 in diameter. A spilce knot is one sawn in a lengthwise position. A 

 dead, or, loose knot is one not firml)' held in place \>y growth or 

 position. 



(4) Pith. At the 

 center or axis of the 

 tree is the pith or 

 medulla, Fig. 34. In 

 every bud, that is, at 

 the apex of everj' stem 

 and branch, the pith is 

 the growing part; but 

 as the stem lengthens 

 and becomes overgrown 

 by successive layers of 

 wood the pith loses its 

 vital function. It does 

 not grow with the 

 plant except at the 

 buds. It varies in 

 thickness, being very 

 small, — hardly moi'e 

 than 1/16", in cedar 

 and larch, — and so 

 small in oak as to be 

 liardly discernible; and 

 what there is of it 

 turns hard and dark. 

 In herbs and shoots it 

 is i-elatively large, Fig. 

 0, p. 1-5, in a three- 

 year old shoot of el- 

 der, for exa7nple, be- 

 ing as wide as the wood. In elder, moreover, it dies early and pul- 

 verizes, leaving the stem hollow. Its function is one of only tem- 

 porary value to the plant. 



Fiy. 34. Section Tbru the Trunk- of a Seven Year 

 Old Tree, Sliowinj^ Relation of Branches to Main 

 Stem. A, B, two branches which were killed after 

 a few years' growth bv shadin!^--, and which have 

 been overg"rown by the annual rin^rs of wood; C, a 

 limb which lived four years, then died and broke 

 off near the stem, leaving- the part to the left of 

 XY a "sound" knot, and the part to the rif^ht a 

 "dead" knot, which unless rotlinj^ sets in, would 

 in time be entirely covered by the growing- trunk; 

 D, a branch that has remained alive and has in- 

 creased in size like the main stem; P, P, pith of 

 both stem and limb. 



