MORPHOLOGY OF WOOD 245 



resistance to stress, and consequently should be used with caution 

 wherever it is likely to develop severe strains. 



The white ash, Fraxinus americana, was chosen as an example of a 

 ring-porous wood, not only because of its importance in industry, but 

 also because of variability in its structure and tendency to be brash. 

 Figure 5 shows a cross-section of a strong white ash timber cut so as 

 to include the end of one annual ring and the beginning of the next. 

 Here, owing to a further specialization of the conducting tissue, the 

 organization is more complicated than that shown in the previously 

 mentioned species. In the early springwood the vessels are large, as 

 can be seen by the portion of one included in the upper part of the 

 picture or even more clearly by those represented in the lower portion 

 of figure 8, in which an entire annual ring is included. In the summer- 

 wood, on the other hand, the vessels are small and more widely sep- 

 arated. Such a restriction of the large conducting elements to one 

 region creates a zone of weakness, a feature which has for a long time, 

 been utilized by basket makers. The rays are of the diffuse multiseriate 

 type quite similar to those appearing in Liriodendron except for cer- 

 tain diagnostic differences. The multiseriate character is even better 

 shown in figures 10 and 11 which represent their contour in tangential, 

 section. Other storage cells, vasicentric parenchyma, surround the 

 vessels and tend to increase the susceptibility to brashness in proportion 

 to their volume. The fibers are of two types. In the springwood they 

 are short, thin-walled, and blunt at the ends, while those in the summer- 

 wood are long, thick-walled, and attenuate, both types of which are 

 shown conventionally in figure 7, and it will be observed that end 

 modifications in the longer summer fibers are very similar to those 

 already mentioned as occurring in tulip poplar. 



As in the two cases mentioned above, thinness of the fiber wall is 

 one of the most important indications of brashness, while the relative 

 proportion of short spring fibers and long summer strengthening cells 

 is of equal importance. This is more clearly shown in figure 5 which 

 represents a strong block of white ash with its thick-walled summer 

 fibers, and in figure 6 which pictures a weak specimen with thin-walled 

 cells in the same region. The more exact differences between these 

 two woods are clearly set forth in Table 3 which shows the following 

 correlations : a specific gravity of .70 is associated with a summer fiber 

 wall thickness of .006, a percentage of ray tissue of 10 and a per- 

 centage of summerwood of 70. The brash one on the contrary shows 



