CONIFEROUS WOOD 



19 



able aid in distinguishing heavy, strong pine wood from that 

 which is Hght 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 hght and dark, and on a longitudinal but tangential 

 section by much broader alternating and less parallel stripes with 

 some V-shaped hues (Fig. 12). 



Under the microscope a transverse section of coniferous secondary 

 wood presents regular straight radial rows of apparently 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 



Fig. 13 —Transverse section, of Spiuce(Picrea exceka), magnified 100 times, showing 

 narrow rings, tliin walls and three ream-dTicts (From Hartig s Timbet s and how to 

 InAo them, toy permission of Di. Somorville and Mr. David Douglas.) 



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 greyish dots, which on being mag- 

 nified are seen to be the cross sections of relatively large spaces, 

 the resin-passages, each surrounded by a layer of thin-walled cells, 

 the resin-epithelium (Fig. 13). These resin-passages are not cells 

 or vessels, but intercellular spaces, into which the resin oozes from 

 the surrounding epithehum (Fig. 14). They generally occur singly, 

 though sometimes in groups, and are most readily detected on a 

 very smooth surface, or are often more easily seen on radial or 

 tangential sections. On these they appear as fine Hnes or scratchea 



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