24 



WOOD AND FOREST. 



(1) Non-porous woods, which comprise the conifers, as pine and 

 spruce. 



(2) Ring -porous woods, in which the pores appear (in a cross- 

 section) in concentric rings, as in chestnut, ash and elm. 



(3) Diffuse-porous woods, in which (in a cross-section) the rings 



are scattered ir- 

 regularly thru 

 the wood, as in 

 bass, maple and 

 yellow poplar. 



In order to 

 fully understand 

 the structure of 

 wood, it is nec- 

 essary to exam- 

 ine it still more 

 closely thru the 

 microscope, and 

 since the three 

 classes of wood, 

 non-porous, ring- 

 porous and dif- 

 fuse-porous, dif- 

 fer considerably 

 in their minute 



structure, it is well to consider them separately, taking the sim- 

 plest first. 



Non-porous woods. In examining thru the microscope a trans- 

 verse section of white pine, Fig. 18 : 



(1) The most noticeable characteristic is the regularity of ar- 

 rangement of the cells. They are roughly rectangular and arranged 

 in ranks and files. 



(2) Another noticeable feature is that they are arranged in belts, 

 the thickness of their walls gradually increasing as the size of the 

 cells diminishes. Then the large thin-walled cells suddenly begin 

 again, and so on. The width of one of these belts is the amount of 

 a single year's growth, the thin-walled cells being those that formed 

 in spring, and the thick-walled ones those that formed in summer, 

 the darker color of the summer wood as well as its greater strength 

 being caused by there being more material in the same volume. 



Fig. 15. Cross-section of JN on-porous Wood, White 

 Pine, Full Size (top toward pith). 



