In Pinus taeda L., burst, breaking length, and sheet density were improved by using fiber refined from wood having long, narrow-diameter tracheids with thick walls. Only narrowdiameter tracheids with thick walls were required to improve tear factor. A theoretical stress analysis revealed that thick-walled cells of small outside diameter fail by diagonal tension or parallel shear, depending on the fibril angle, while under torsional stress during refining. Such failures result in ribbon-like fragments which research elsewhere has demonstrated to provide the coherence necessary for strength development in mechanical pulps. In contrast, thinwalled cells of small outside diameter fail by diagonal compression and yield few ribbons. Long fibers are more desirable than short fibers because of greater induced stresses and improved chances of axial alignment between the disks. © 1969 Springer-Verlag.
