Hardwood vessels

In hardwoods, morphological structural elements in longitudinal series comprise the segmented structure termed vessel . Vessels, which are exposed in transverse section, constitute about 10-46% of the stem volume in deciduous hardwoods and are cells of relatively large diameters (50-300 p.m). Vessels have in short the appearance of open vertical tubes within the wood structure because their end walls have partially dissolved. By comparison, the hardwood vessel diameter can be as much a 10 times the diameter of a softwood fiber. 

Trees are classified into two major groups termed softwoods (gymnosperms) and hardwoods (angiosperms). The botanical basis for classification is whether or not the tree seed is naked as in softwoods or covered as in hardwoods. A more familiar classification, which with some exceptions is valid, is based on the retention of leaves by softwoods or the shedding of leaves by hardwoods. Thus the softwoods are often referred to as evergreen trees and hardwood as deciduous trees. The major difference with regard to wood anatomy is the presence of vessels in hardwoods. Vessels are structures composed of cells created exclusively for the conduction of water. Softwoods lack vessels but have cells termed longitudinal tracheids which perform a dual role of conduction and support. 

Muller M (1980) Beitrag zum Einsatz der Curie Punkt-Pyrolyse Massenspektrometrie in der Analytik von Naturstoffen Dissertation ETH 6743, Zurich Obst JR (1983) Analytical pyrolysis of hardwood and softwood lignins and its use in lignin type determination of hardwood vessel elements J Wood Sci Technol 3 377-397 Pouwels AD, Boon JJ (1987) Analysis of lignin and chlorohgntn residues in a beech xylan fraction by pyrolysis gas chromatography mass spectrometry J Wood Sci Technol 7 197— 213... 

Blanchette RA, Obst JR, Hedges Jl Wcliky K (1988) Resistance of hardwood vessels to degradation by white rot Basidiomvcctes Can J Bot 66 1841-1847... 

Figure 26. SEM of major types of perforation plates in hardwood vessel elements. (A) Wood radial section of redgum showing scalariform (ladderlike) verjoration plates in three vessel elements (RP = ray cross-field pits). (B) Portion of a maple vessel element containing a simple perforation plate chemically pulped wood. (C and D) Portions oj vessel elements from yellow poplar (C) and paper birch (D) showing scalariform plates with very few bars and with numerous bars chemically pulped wood.

Better runnability and fewer wet-end breaks Improved filler retention and better filler distribution Filler retained by cationic starch is also bonded in the web Reduced linting and dusting Reduced hardwood vessel picking Reduced pick outs... 

Wood also contains 3—10% of extraceUulat, low molecular weight constituents, many of which can be extracted from the wood using neutral solvents and therefore ate commonly caUed extractives. These include the food reserves, the fats and their esters in parenchyma ceUs, the terpenes and resin acids in epitheUal ceUs and resin ducts, and phenoUc materials in the heartwood. Resin materials occur in the vessels of some hardwood heartwood. 

The basic structure of all wood and woody biomass consists of cellnlose, hemicelluloses, lignin and extractives. Their relative composition is shown in Table 2.4. Softwoods and hardwoods differ greatly in wood stmctnie and composition. Hardwoods contain a greater fraction of vessels and parenchyma cells. Hardwoods have a higher proportion of cellulose, hemicelluloses and extractives than softwoods, but softwoods have a higher proportion of lignin. Hardwoods ate denser than softwoods. 

Lignin in the true middle lamella of wood is a random three-dimensional network polymer comprised of phenylpropane monomers linked together in different ways. Lignin in the secondary wall is a nonrandom two-dimensional network polymer. The chemical structure of the monomers and linkages which constitute these networks differ in different morphological regions (middle lamella vs. secondary wall), different types of cell (vessels vs. fibers), and different types of wood (softwoods vs. hardwoods). When wood is delignified, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived. 

Figure 5. Cross-sectional and longitudinal surfaces of a ring-porous hardwood. In the cross-sectional view (X) the largest diameter cells are springwood vessels whereas the smaller cells with obvious openings are sum-merwood vessels. Smaller diameter thick-walled fibers constitute most of the remaining volume. Transversely oriented food-storing cells can be seen on the radial surface (arrow). 40X (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environmental Science and Forestry)...

Obviously, softwood anatomy is relatively simple as only two types of cells, longitudinal tracheids and ray parenchyma, constitute the bulk of the wood. Hardwoods have a more complex anatomy as more kinds of cells are present. The roles of conduction and support are carried out by different cells and in addition to the transverse ray parenchyma, food-storage cells oriented in the longitudinal direction are present. Parenchyma oriented longitudinally are called longitudinal or axial parenchyma. Vessel segments perform the conduction role, and fibers the support role. 

Thus, most hardwood species contain four types of cells, vessel segments, fibers, transverse and axial parenchyma, whereas most softwood species possess two types longitudinal tracheids and transverse parenchyma. 

When vessels end, they rarely do so in isolation but rather among a group of vessels. Translocation continues into the adjacent vessels via the intervessel pits. These pits differ from softwood bordered pits in that they lack a torus and openings large enough to be readily detected with an electron microscope. Figure 17 depicts a typical intervessel pit membrane. Different arrangements of intervessel pits can be detected and are useful in the identification of hardwood species. 

Figure 16. Types of vessel segments found in hardwoods. A and B springwood vessels from a ring-porous wood. Note the short length compared with the diameter. C and D typical vessel elements from diffuse-porous woods with simple perforation plates at each end. E typical diffuse-porous vessel element with scalariform perforation plates at each end. 140X...

Based on the wood anatomical descriptions presented, it is obvious that hardwoods and softwoods differ considerably from each other. For example, vessels are present in hardwoods and absent in softwoods. In hardwoods more cell types, shorter cells, more parenchyma and a more variable arrangement of cell types occur. The relative uniformity of softwood anatomy is the result of the preponderance of a single type cell, the longitudinal tracheid. 

At the beginning of the growth the tree requires an effective water transportation system. In softwoods thin-walled cells with large cavities are formed in hardwoods special vessels take care of the liquid transportation. Comparatively light-colored and porous earlywood is thus formed. Later, the rate of growth decreases and latewood is produced. It consists of thick-... 

Water conduction in a tree is made possible by pits, which are recesses in the secondary wall between adjacent cells. Two complementary pits normally occur in neighboring cells thus forming a pit pair (Fig. 1-5). Water transport between adjacent cell lumina occurs through a pit membrane which consists of a primary wall and the middle lamella. Bordered pit pairs are typical of softwood tracheids and hardwood fibers and vessels. In softwoods the pit membrane might be pressed against the pit border thus preventing water transport, since the torus is impermeable. The pits connecting tracheids, fibers, and vessels with the ray parenchyma cells are half-bordered. Simple pits without any border connect the parenchyma cells with one another. 

Hardwoods contain several cell types, specialized for different functions (Fig. 1-9). The supporting tissue consists mainly of libriform cells, the conducting tissue of vessels with large cavities, and the storage tissue of ray parenchyma cells. In addition, hardwood contains hybrids of the above-mentioned cells which are classified as fiber tracheids. Although the term fiber is frequently used for any kind of wood cells, it more specifically denotes the supporting tissue, including both libriform cells and fiber tracheids. In birch these cells constitute 65 to 70% of the stem volume. 

The hardwood resin is located in the ray parenchyma cells which are connected with the vessels. It consists of fats, waxes, and sterols. The accessibility of the resin depends on the pore dimensions as well as on the mechanical stability of the ray parenchyma cells. Considerable variations occur among different wood species (Table 5-3). For instance, the accessibility of the resin in birch is much lower than in aspen. 

Wood can be considered as a biological composite of hollow tubes of cellulose fibers held together by a lignin matrix gluing material. Liquids are transported up and down the trees through the tubular plumbing system. About 90 percent of the wood tissue of softwood trees consists of fiber tracheid cells for liquid conduction and support. The hardwood trees evolved after the softwoods and have specialized water conduction cells called vessels. 

Virtually all cavities of wood cells are interconnected, as mentioned above for the vessel members of hardwoods. Tiny openings in the cell walls of overlapping ends of tracheids provide vertical passageways for water rising from cell to cell in conifers. Thin walls of ray cells are perforated for the radial transport of sugars, and tangential connections provide for... 

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