CONIFEROUS WOOD. 21 



(0'7 mm.), an average for the twenty outermost rings, and even 

 a minimum of one two-hundredth of an inch (0'2 mm.). 1 Many 

 local causes, especially exposure to wind, produce excentricity 

 of growth, few trees presenting a truly circular cross-section or 

 a truly central pith, though this is more common among pines 

 than among other trees. Branches almost always present an 

 excentrically oval section, the pith nearer to the upper surface. 

 The summer- wood in each ring being darker, heavier, and denser, 

 its relative proportion to the spring-wood largely determines the 

 weight and strength of the wood, so that colour becomes a 

 valuable aid in distinguishing heavy, strong pine wood from that 

 which is light and soft. Whilst on a cross-cut or transverse 

 section the annual growths appear as rings, on a longitudinal 

 radial section they are represented by narrow parallel stripes 

 alternately light and dark, and on a longitudinal but tangential 

 section by much broader alternating and less parallel stripes 

 with some V-shaped lines (Fig. 12). 



Under the microscope a transverse section of coniferous 

 secondary wood presents regular straight radial rows of appar- 

 ently four-sided meshes or openings, the transverse sections of 

 tracheids. These are as broad in a radial as in a tangential 

 direction in the spring wood, but much narrower radially in the 

 summer wood of each ring. The cell-walls also are thicker in 

 the summer wood. The radial walls have bordered pits, and in 

 some cases such pits also occur on the tangential walls. Scattered 

 through the summer wood are numerous irregular grayish dots, 

 which on being magnified are seen to be the cross sections of 



1 Poplars grown in moist ground may reach a diameter of 14 inches in 

 8 years. Laslett records (Timber and Timber-trees, ed. 2, pp. 44-5) 

 exceptionally fine English Oak and Elm, and an average drawn from several 

 specimens of Canadian Oak and Elm which gave the following number of 

 rings at 6, 12, 18, and 24 inches diameter : 



6 in. 12 in. 18 in. 24 in. 



English Oak, 13 19 24 30 



Canadian Oak, 49 105 160 216 



English Elm, 10 16 25 36 



Canadian Elm, 80 156 252 



