OBSERVATIONS ON CANADIAN CONIFEEiE. 



39 



No. VIL 



VARIATIONS OF TRACHEIDS. 



From this it is evident tliat, while the average size of a spring tracheid is twice that of 

 a summer tracheid, there is no constancy in these relations even in the same tree, where the 

 variation may he stronly marked as shown hy a comparison of 35rt and 36b. And ao-ain it 

 may he noted that there is no api)arent relation between such variations and diflerences in 

 weight or coefficient of elasticity, and this is rendered more conspicuous when we observe that 

 in No. 2, which represents the highest grade of this timber, there is the greatest difference 

 in the size of the tracheids. 



From this it would appear justifiable to conclude that the strength of material is in no 

 way influenced by the relative dimensions of the spring and summer tracheids. 



The peculiar frequency and direction of fractures in certain varieties of this wood have 

 already been referred to. The tangential fractures produced as a result of seasoning, are 

 found to arise immediately external to the dense summer wood and to follow this structure 

 closely, so that a clean, hard surface is presented. An examination of the line of fracture 

 shows that it (fig. 1, plate III.) extends directly across the large open tracheids of the first 

 formed spring wood. This, then, is in no sense a cleavage line, but one of mechanical rupture 

 as the result of uiiecpial contraction in the contiguous layers of spring and summer wood. 



If we ask why the coarse-grained woods shake in this manner, while the fine-drained 

 varieties do not, the answer is one which cannot be based upon simple structural variations. 

 Thus, if we compare No. 2 and No. 316, the two specimens which exhibited the widest dif- 

 ferences with respect to the development of shakes, we find that in each case the thickness 

 of the tracheid walls is the same, while there are in other respects no structural variations 

 which would serve to account for their different behaviour under conditions of stress or of 

 seasoning. These relations may be seen in the annexed table : 



No. VIII. 



THICKNESS OF TRACHEID WALLS. 



