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THE POPULAR SCIENCE MONTHLY. 



cation of the species, and at the same time enable one to judge of the 

 suitability of a particular wood for definite uses. 80 little has been 

 done in this country in the microscopical study of the woods for engi- 

 neering, architectural, or mechanical purposes, that but few are aware 

 of the variety in form and structure of the wood cells, ducts, and 

 special fibers which make up the woody tissue of the different species. 

 An expert can readily determine whether a certain wood, used for rail- 

 road-ties, will sustain the service of a trunk line, or is only suitable 

 for a branch of limited traffic. 



In the Coniferce, which includes the pines, cedars, larches, red- 

 woods, spruces, and firs, as a rule, each layer of growth only has two 

 kinds of wood-cells called tracheids, one of thin walls and a large lumen, 

 and the other of thick walls and a small lumen ; when the former pre- 

 dominates, making nearly all of the layer, the wood is generally soft, 

 as in the white pine (Pinns strobus, L.), the cedars, redwoods, spruces, 

 and firs. When the thick-walled cells form one fourth to one half of 

 the layer, the wood is much harder, as in the long-leaf yellow pine 

 (Pinus palustris, Mill), Pinus mitis, and the larches. On the thin- 



Fig. 1. Transverse Section of Pinus palustris Fig 2. Transverse Section of Chamcecy- 



(Mill), -\- . parts sphcerorda (Spalch), -\- (White 



Cedar). 



walled cells of all the species of the Coniferce are dome-like or lenticu- 

 lar markings, principally on the sides parallel to the medullary rays. 



The thick-walled cells are often marked on the sides at right an- 

 gles to the medullary rays. The Coniferce have more or less resinous 

 products, and the presence or absence of the upright resin-canals aid 

 in distinguishing the genera, while the form and character of the 

 medullary rays, the presence or absence of resin-ducts, the character 

 of the cells, enable the species to be identified. In the alburnum or 

 sap-wood, the starch is confined to the cells around the resin-canals 

 and in the cells of the medullary rays. 



The cellular structure of the oaks, chestnuts, hickories, ashes, wal- 

 nuts, maples., beeches, birches, and magnolias is far more complex and 



