MORPHOLOGY OF WOOD 243 



rays fall in a direction perpendicular to the long axis, and the indi- 

 vidual cells split rather than pull apart at the point of union with a 

 neighboring cell. In a brash wood, therefore, it would appear that the 

 vessels are especially weak since they are the first to buckle and break 

 under the strain. The ray cells offering a line of least resistance at 

 right angles to that of the other elements tend toward weakness in 

 proportion to the amount of space they occupy. The fibers, the last to 

 give way, in weak woods split through the walls and the resistance 

 they offer depends largely upon the amount of woody substance pres- 

 ent. The broken surfaces illustrated in figure 12 show a break char- 

 acteristic of a strong wood. Here only a small proportion of the ele- 

 ments are broken transversely as they are in the brash one, but rather 

 in a somewhat diagonal-vertical line so as to form numerous small 

 splinters. As in the first case vertical rupture is usually through the 

 vessels, whose thin walls offer a low resistance to flexure or compres- 

 sion. Failure along the rays is similar to that already described. The 

 fibers are as usual the last to break in strong woods, and tend to 

 separate rather than to split through the wall, and, as before, pulling 

 apart at the ends is rare. Whenever the ends are broken the line of 

 rupture is for the most part diagonal and many were seen where the 

 ends were much frayed. Observations similar to these are recorded by 

 Brush in his paper on the anatomy of mechanical failure.^ Since 

 failure appears to follow the vessels and rays more generally than 

 it does the fibers, it seems safe to assume that the proportional amount 

 of ray and conducting tissue in diffuse porus woods are among the 

 most important factors in determining strength or brashness. 



A photomicrographic transverse section of a strong tulip poplar, 

 Linodendron titlipifcra, is shown in figure .3, and it will be observed 

 that the fibers are thick-walled and occupy a large proportion of the 

 volume. The vessels are small and well separated but the rays, unlike 

 those illustrated for the bald cypress, tend to spread at the juncture 

 of the two annual rings and are in general of the diffuse multiseriate 

 type. A band of thick-walled parenchyma appears at the end of the 

 ring but it is doubtful if this narrow plate of storage tissue has much 

 effect upon strength as in neither case where broken specimens were 

 examined did rupture seem to follow the end of the annual zone. 

 Figure 4: shows a similar section of a brash tulip poplar, taken from 



' Brush, W. D. A Microscopic Study of the Mechanical Failure of Wood. 

 Review of Forest Service Investigations, Vol. 2, 33-42, 1913. 



