MORPHOLOGY OF WOOD 247 



the large size and general distribution of the vessels and upon a higli 

 percentage of the wood rays. To be sure, other structural features 

 have their influence, such as the diameter of the fibers, and the amount 

 of vasicentric parenchyma around the vessels, but in general their 

 control is slight in comparison with those just listed. 



The above observations have shown that anatomical variations play 

 an important role in determining strength or weakness in wood. In 

 the coniferous group, the relative proportion of the thick-walled sum- 

 mer tracheids, and the thin-walled spring cells is of the utmost im- 

 portance. A large percentage of late tissue is indicative of strength 

 and for this reason the yellow pines are structural timbers par excel- 

 lence. In the same way the small band of thick-walled tracheids at 

 the end of the annual ring in the redwood militates against its use 

 where a high resiliance is necessary. Very closely associated with the 

 proportion of summerwood and springwood, is the thickness of the 

 tracheid wall, which has been shown to be roughly proportional to 

 specific gravity. Two other structures which always reduce strength 

 as they increase in number or volume are the storage rays and the 

 bordered pits. The rays are not as important in the softwoods as they 

 are in the hardwoods, since they are practically always uniseriate in 

 the Pinaceous type, and the volume of the tissue specialized for con- 

 servation is always small. The bordered pits are more important, since 

 they form actual holes in the wall and upon failure are regions of 

 primary rupture. Consequently the greater the size and number of 

 the bordered pits the greater will be the susceptibility to brashness as 

 was numerically shown in the case of the bald cypress. 



In the dififuse-porous woods, the number and distribution of the 

 fibers has perhaps the greatest influence on strength. The stronger 

 woods show a high volumetric per cent of strengthening cells and a 

 correspondingly smaller area occupied by vessels and rays. The 

 thickness of the fiber walls is of equal importance, since it determines 

 the amount of woody substance which of course is the ultimate test 

 for strength or brashness. The influence exerted by the rays and other 

 tissues depends upon their volume and naturally increases the liability 

 to failure in proportion to the space they occupy. Brashness in diffuse- 

 porous woods, therefore, depends primarily upon the number and size 

 of the vessels, upon the volume of fibers, upon the thickness of the 

 fiber walls, and upon the number and size of the rays and other 

 parenchymateous regions. 



