THE STRUCTURE OP WOOD. 



23 



The substance of which they are composed is "parenchyma" 

 (Greek, beside, 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 ray, 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 



As the tree 

 grow s larger, 

 new, or second 

 ary medullary 

 ray? start from 

 the cambium 

 then active, so 

 that every yeai 

 new rays are 

 formed both 

 thinner a n d 

 shorter than the 

 primary rays, 

 Fig. 14, 



iSTow suppose 

 that laid among 



the ordinary thin-walled tubes were quite large tubes, 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 tubes would be called vessels, or tracJiece. Suppose again that 

 these large tubes were scattered in disorder thru 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. 15, oak or ash, Fig. 16, 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." Bv this classification we have: 



. 14 



Cross section of White Oak. The Radiating- White 

 Lines are the Pith Ra3 T s. 



