TIIK STRUCTUEE OP WOOD. 



23 



The substance of ■which the}- are composed is "parencliyma" 

 (Greek, I/eside, to pour), which also constitutes the pith, the rays 

 forming a sort of connecting link between the first and last growth 

 of the tree, as the cambium cells form new wood each year. 



If a cambium cell is opposite to a pith I'ay, it divides crosswise 

 (transversely) into eight or ten cells one above another, which 

 stretch out radi- 

 ally, retaining 

 their protoplasm, 

 and so continue 

 the pith ray 



A.s the tree 

 grows larger, 

 new, or second- 

 ary medullary 

 rays start from 

 the cambium 

 then active, so 

 that every year 

 new ravs are 

 formed both 

 thinner a n d 



shorter than the 

 primary ra}"s. 

 Fig. 14. 



Now suppose 

 that laid among 



the ordinary thin-wallcd Lujjcs were quite laige tulies, so that one 

 could tell the "ring" not only by the thin walls but by the presence 

 of large tubes. That would represent the ring-porous woods, and the 

 large tuljes would be called vessels, or tnirlien'. Suppose again that 

 these large tubes were scattei-cd in disorder thin the layers. This ar- 

 rangement would represent the diffuse-porous woods. 



By holding up to the light, thin cross-sections of spruce or pine. 

 Fig. ]o, oak or ash. Fig. 10, and bass or maple. Fig. 17, these three 

 quite distinct arrangements in the structure may be distinguished. 

 This fact has led to the classification of woods according to the 

 presence and distribution of "pores," or as they are technically called, 

 "vessels" or "tracheae."' By this classification we have: 



Fif. 14 Cross stction of White Oak. The Radiatiiif,' White 

 Lines ai'e the Pitli Rays. 



