1 68 



THE STEM PROPER 



durable heartwood so prized by lumbermen. The heart- 

 wood is useful to the plant principally in giving strength 

 and firmness to the axis. It will now be seen why gird- 

 ling a stem, that is, chipping off a ring of the softer parts 



all round; will kill it, while • 

 we often see vigorous and 

 healthy trees with the cen- 

 ter of the trunk entirely 

 hollow. 



237. Vertical Arrange- 

 ment. — In studying the 

 vertical arrangement of 



311 3'2 3"3 . . .• 



stems two sections are nec- 



311-313. — Diagrams of sections of j* i j 



timber : 311, cross section ; 312, radial ; essary, a radial and a taU- 



313, tangential (from PiNCHOT, U.S. orential One. The former 



Dept. of Agr.). ° , , . ,. 



passes along the axis, spht- 

 ting the stem into halves (Fig. 312); the latter cuts 

 between the axis and the perimeter, splitting off a segment 

 from one side (Fig. 313). 



238. The Graining of Timber. — It is the medullary rays 

 that constitute the characteristic graining of different 

 woods. In a chip of red oak or chestnut from just beneath 



314. — Tangential section of mountain ash, showing ends of the medullary rays. 



the bark their cut ends can be seen very distinctly with the 

 naked eye. Split a thicker chip of the same kind parallel 

 with the medullary rays and notice the difference, the 

 rays now appearing as silvery bands traversing the wood. 



