40 



D. P. PENHALLOW ON 



Radial fractures resulting from mechanical stress or from seasoning arise inclejjendently 

 of the medullary rays, toward which they often take an oblique direction. It is thus evident 

 that these rays do not present lines of greatest structural weakness, and in this the genus 

 Pseudotsuga, as is also the case with some specimens of Larix occidentalism presents a notable 

 exception to the general law of fracture. Inasmuch as these fractures are of the same nature 

 in Pseudotsuga and Larix occidentalis, they may doubtless lie referred to the same cause, 

 i.e., organic weakness in the sulistance of the primary cell wall. That coarse-grained woods 

 develop these peculiar fractures freely, while the fine-grained woods do not, serves as an 

 indication that these differences depend largely, if not wholly, upon climatic or other condi- 

 tions of growth, whereby, probably, different volumes of the elements of water enter into 

 the organization of the cell membrane which, in consequence, exhibits various degrees of 

 brittleness or susceptibility to rupture. This view appears to gain support from the import- 

 ance wbicli lumbci-nieu attach to loctdity as intlueiiciiig the (piality of the timlier. 



The longitudinal shear referred to as produced under the inilueuce of mechanical stress 

 is only an exaggerated form of the star and cup shake. The lirst line of rupture appears to 

 1)e established through the spring wood close to its junction with the summer wood of the 

 ])revious year. As the two sei)arate, the latter is left with a smooth, hard surtace. The line 

 of fracture thus established is in all essential respects the same as that which arises in the 

 same position as a result of shrinkage, but it has the peculiarity of exhibiting certain areas 

 of resistance to fracture. These aretis of resistance arc found in the form of triangular elevii- 

 tions disposed in a row of considerable length, and such rows arise at frequent intervals 

 throughout the length of the fracture. (Plate IV., tig. 3.) An examination of the surface of 

 one of these elevations shows the tracheids to have been ruptured as in the case of cup shakes, 

 with the difference that the line of fracture is developed at an angle of about do"" to the 

 radius, as shown in fig. 2, plate III. 



The only otlier structural feature meriting consideration in this connection is to be found 

 in the size and distribution of the resin-passages. Striking variations in both of these 

 respects are to be noted, but their bearing upon the timber for economic purposes must relate 

 wholly to durability as dependent upon or influenced by the relative amount of resin present. 

 An examination of the various specimens of Douglas fir so fiir Ijrought under notice, has 



given the followina; results : 



^ No. IX. 



Variations in Resin-passages. 



