Trees — The Character, Structure, &c. 



53 



Pith Flecks. — Peculiar patches which have a pathologic 

 origin are sometimes found in wood, and usually appear coloured 

 and well defined on the cross cut section of logs, or on tangential 

 boards. In Hornbeam and Cherry they are inconspicuous, owing 

 to the colour of the wood. They frequently occur in Willow, 

 Poplar, Alder, Birch, Lime, and many trees belonging to the 

 Rosacece. They are the burrows caused by the larvae of a 

 dipterous insect (Tipula) feeding upon the cambium during the 

 growing season, and afterwards becoming occluded with wound 

 tissue of a soft cellular nature. (Fig. 3). 



Surfaces of Timber. 



In cutting timber there ire three surfaces or exposures, and 

 in order to understand the structure of timber, it is important to 

 study the three planes of section : (Fig. 4) (l) the transverse 

 or cross cut section, i.e. at right angles to the long axis ; on this 

 surface the annual layers appear as concentric rings and the rays 

 as long lines as it shows the latter in longitudinal view from 

 above or below : (2) the radial cut or quarter-sawed timber, 

 i.e. cut towards the centre of the log parallel with the 

 rays : the rings then appear as vertical lines, and crossing these 

 at right angles are the wood rays disi)laying their longitudinal 

 view from the side to form flat expanded surfaces known as 

 silver grain in large rayed woods : (Fig. 5) (3) the tangential 

 surface or slab cut, which is parallel to the bark and at right 

 angles to the rays. Here the rays are not conspicuous, but under 

 a lens appear as short pointed streaks distributed irregularly 

 among the other tissue, this is a very important surface, however, 

 when studying timber, because we get some idea as to the height 

 and breadth of rays as we view them in transverse section. The 

 rings appear on this surface as ellipses or hyperbolas in the 

 centre and open stripes at the side. To grasp this, one must 

 remember that the rings are hollow cones nested one within 

 another. 



