WOOD IN GENERAL 



251 



the closed longitudinal tubes that form the main part of 

 wood, cannot be all just alike for they occur in alternating 

 layers of darker and lighter color. Examination of these 

 layers under the magnifier shows that in the lighter colored 

 laj^er the tubes are of decidedly larger bore than those form- 

 ing the darker layer. The pale layers of less compact ma- 

 terial are called spring wood, and the more compact layers, 

 summer wood, for reasons that will presently appear. On 

 a tangential surface, that is to say, one cut with the grain 

 but not toward the center of the log, these contrasted layers 



Fig. 230. — Tangential section of 

 white pine wood, V. (Original.) 



Fig. 231. — Transverse .section of 

 white pine wood, Y. (Original.) 



appear as broader bands often in beautiful .systems of curves. 

 On such a surface the cut ends of the pith-rays are to be 

 seen under the magnifier (Fig. 230) as small, verj- narrow 

 spots or streaks. If now we examine a crosscut or transverse 

 section (/. c, a thin slice made at right angles to the direction 

 of the fibrils) the magnifier will show us something more of 

 the form and arrangement of the parts. As shown in Fig. 231 

 we can look through the central cavities of the tubular fibrils, 

 and so get a better idea of their sizes and shapes and the 

 thinness of the walls. They are seen to be arranged in radial 

 rows, between which the pith rays often appear as more or 

 less delicate lines of dense material. Here and there among 



