120 



PRACTICAL COURSE IN BOTANY 



usually consists of sapwood, which is soft and worthless as 

 timber, while the dead interior forms the durable heart- 

 wood so prized by lumbermen. The heartwood is useful to 

 the plant principally in giving strength and firmness to the 

 axis. It will now be seen why girdling a stem, that is, chip- 

 ping off a ring of the softer parts all round, will kill it, while 

 vigorous and healthy trees are often seen with the center of 

 the trunk entirely hollow. 



132. Different ways of cutting. In studying the vertical 

 arrangement of stems, two sections are necessary, a radial and 

 a tangential one. The former passes along the axis, splitting 

 the stem into halves (Fig. 135) ; the latter cuts between the 



axis and the perimeter, split- 

 ting off a segment from one 

 side (Fig. 136). The appear- 

 ance of the wood used in car- 

 pentry and joiner's work is due 

 largely to the manner in which 

 the planks are cut. 



133. The cross cut. The 

 section seen at the end of a log 

 (Figs. 132, 134) is called by 

 carpenters a cross cut. It 

 passes at right angles to the 

 grain of the wood, and severs what important structures? 

 (116, 119, 122.) Examine a cross cut at the end of a rough 

 plank, or the top of 

 a stump or an old 

 fence post, and tell 

 why this kind of cut 

 is seldom used in 

 carpentry. 



134. The tangent ^ 



cut is so called be- 



cause it is made at '^^-7an^^ 



right angles to the ing ends of the medullary rays. 



134 135 136 



FIGS. 134-136. Diagrams of sec- 

 tions of timber: 134, cross section ; 

 135, radial; 136, tangential. (From 

 PINCHOT, U. S. Dept. of Agr.) 



