420 MR ANGUS R. FULTON ON EXPERIMENTS TO SHOW 
(b) Cause of Characteristic Faulure. 
The reason why the buckling of the wood fibres invariably takes place in a 
tangential direction is better seen on examining the section of the timbers under 
the microscope. 
The normal appearance of Oak when examined thus is shown by the micro-photo- 
graphs figs. 7, 8,9, and 10. Fig. 7 is a normal radial section showing a side view of 
a medullary ray seven cells deep. Fig. 8 is a normal transverse section at zone of 
demarcation between autumn wood of one year and spring wood of the next. Two 
narrow or secondary medullary rays are shown in plan, and appear as single rows of cells. 
The more general appearance of the normal transverse section is given in fig. 9, where 
the magnification is less and a larger portion of the section is seen. The great size 
of the tracheide compared with that of the wood fibres should be kept in mind for 
comparison with the section, say, of Boxwood. 
But for our purpose possibly the most instructive section is that of fig. 10, in which 
a normal tangential section is given, showing cross-sections of the cells of the medullary 
rays, broad and narrow, with wood fibres and parenchyma running longitudinally and 
sinuously between the groups of ray cells. 
If the cross-section of the trunk of a tree is viewed superficially, it might be 
regarded as made up of a number of thin concentric cylinders continuous throughout 
the length of the tree and all glued together. Each of these thin cylinders represents 
an annual ring which is made up of wood fibres principally, the medullary rays dividing 
it radially, like the jomts between the staves of an ordinary barrel. When subjected to 
longitudinal compressive stress, it would seem as if any buckling that took place in the 
wood fibres would be in a radial direction. 
But, as will readily be seen from fig. 10, the joints, that is the medullary rays, are 
not continuous throughout the length of the tree, nor even for a very small portion of 
it. From the radial section of fig. 7 we see that the wood fibres appear perfectly 
straight and vertical; but from the tangential section, fig. 10, we learn that they have 
a sinuous displacement in the tangential plane, zigzagging their vertical path round 
the cells of the medullary rays. This amounts to an “initial set” in a column 
subjected to end pressure, and failure consequently ensues in the plane containing 
that initial set. 
This is well illustrated in the micro-photographs of wood tested to destruction in 
this way. Fig. 11 is a general view of a tangential longitudinal section of fractured 
Oak, and shows that, though failure has occurred to a small degree at the apparently 
weaker trachez, the principal yield has been along a line where the cohesion between 
the wood fibres and the medullary rays has been the only resistance to the tendency of 
the fibres to buckle. This is shown to a greater magnification in fig. 12, where the 
wood fibres are clearly seen leaving the medullary rays. 
