3098 Chapter 25 



MORPHOLOGY 



The anatomy of the hardwoods that are the subject of this text can be quickly 

 grasped from scanning electron micrographs of small wood cubes (fig. 5-21 

 through 5-45). Morphology of individual hardwood fibers is illustrated in fig- 

 ures 5-53AB. Average stemwood fiber length is 1.27 mm, much shorter than 

 that of the southern pines which average about 4 mm in the four major species. 

 There are significant differences among the pine-site hardwood species, e.g., 

 black tupelo fibers are about 1.76 mm long, while those of red maple are only 

 0.83 mm long (table 5-4). Transverse dimensions of fibers in solid wood (table 

 5-7) are primary indicators of wood specific gravity, but also give some indica- 

 tion of probable properties of sheets formed of pulped fibers. Fibers comprise 

 about 44.4 percent of the stemwood volume of small pine-site hardwoods (41.3 

 percent in the 11 oaks and 47.4 percent in the non-oaks (table 5-3). Vessel 

 elements, comprising about 20.5 percent of stemwood of small pine-site hard- 

 woods, are much larger in diameter than fibers, have thinner walls, are shorter, 

 and are more or less open-ended (figs. 5-52 bottom and 5-99 with related 

 discussion). 



Parenchyma cells (figs. 5-22 and 5-51), including ray parenchyma (figs. 5-56 

 through 5-61), comprise on average about 35. 1 percent of stemwood of small 

 pine-site hardwoods (table 5-3); when these hardwoods are pulped, the paren- 

 chyma cells become fines, i.e., particles so small they are detrimental to sheet 

 bursting and tensile strength. 



Readers needing fiber and vessel micrographs of species additional to those 

 illustrated in chapter 5, will find useful Cote's (1980) atlas of papermaking 

 fibers; the atlas illustrates 24 softwoods, 40 hardwoods, and 10 miscellaneous 

 plants. 



The morphology of pulped hardwood cells is related to properties of sheets 

 made from them. Figure 25-9 illustrates the anatomy of a diffuse-porous hard- 

 wood after removal of most lignin, but before cell separation. When pulped 

 fibers, vessel elements, and parenchyma cells are separated, dispersed in water, 

 and then formed on a screen to yield a paper sheet, they form a random network 

 (fig. 25-10). Mechanical properties of the sheet vary with the morphology of the 

 pulped cells comprising the network, and with other factors including cell 

 hemicellulose and lignin content, beating procedures after pulping, additives to 

 the pulp, forming technique, and sheet drying procedures. Vessel elements bond 

 poorly in these fiber networks and may be picked from paper surfaces by ink 

 application rolls; Byrd et al. (1967) found that gyratory refining of hardwoods 

 pulps at high consistency disintegrated vessel elements, improving internal bond 

 and pick resistance in offset press papers. 



Amidon ( 1 98 1 ) reviewed the effects of wood properties of hardwoods on kraft 

 paper properties. He noted that fiber flexibility (sometimes expressed as lumen 

 diameter/exterior fiber diameter) is a key to chemical pulp quality because 

 flexible fibers have more interfiber bonds than stiff fibers. Long fibers yield 

 papers with greater tear strength than short fibers, until strength of bonds ex- 

 ceeds strength of fibers. Increases in specific gravity generally increase tear 



