KEY TO COMMON WOODS. 



In the following key only such characters are used as may be 

 observed with the naked eye or with a hand lens magnifying twenty 

 diameters. But it is necessary that the observations be made on 

 clean-cut surfaces or on thin, translucent sections made with a very 

 sharp knife. It is often of advantage to moisten the svirface. The 

 end view or Irannverse section, i. e. one directly across the grain, 

 displa3's usually the most characteristic features. The radial sur- 

 face which lies in a longitudinal plane passing through the centre 

 of the log affords also a significant view, while sometimes a tangen- 

 tial surface, or one at right angles to the radial and transverse 

 planes, deserves special notice. 



In transverse section most woods show concentric zones or layers, 

 each of which commonly represents a year's growth, and is accord- 

 ingly known as an annual ring. Each ring consists of two layers, 

 differing more or less in density and color. The part first formed, 

 and therefore on the concave side of the ring, is less dense and 

 commonly lighter in color than the wood formed later in the season 

 when the greater pressure of the bark induces compactness. These 

 layers are known respectively as the spring and the summer wood. 



A certain number of years after it is first formed, the wood in a 

 tree becomes distinctly dryer, more solid, stronger and more 

 durable. It is then called heart-wood. The yoimger, i. e. the outer, 

 part being still comparatively rich in sap is called sap-wood. The 

 two sorts of wood may commonly be distinguished by a marked 

 and abrupt change of color. 



In all true woods there are found numerous thin plates of more 

 or less hardened pith extending in radial planes. These, known as 

 pith rays, often produce beautiful and characteristic markings on 

 radial or nearly radial surfaces, as in " quartered oak." 



Between the pith rays, running lengthwise and forming the main 

 body of the wood, are innumerable, often very minute, short, closed 

 tubes, — the wood fibrils. In the spring-wood their cavities may 

 often be seen as exceedingly small, regularly arranged openings, 



