1 Basic Wood-Moisture Relationships.- 1.1 Introduction.- 1.2 Saturated Vapor Pressure.- 1.3 Relative Humidity.- 1.3.1 Use of the Psychrometric Chart.- 1.3.2 Measurement of Relative Humidity.- 1.3.3 Control of Relative Humidity.- 1.4 Equilibrium Moisture Content and the Sorption Isotherm.- 1.5 The Effect of Changes in Pressure and Temperature on Relative Humidity.- 1.6 Specific Gravity and Density.- 1.7 Specific Gravity of the Cell Wall and Porosity of Wood.- 1.8 Swelling and Shrinkage of the Cell Wall.- 1.9 Swelling and Shrinkage of Wood.- 2 Wood Structure and Chemical Composition.- 2.1 Introduction.- 2.2 The Cell Wall.- 2.3 Structure of Softwoods.- 2.4 Types of Pit Pairs.- 2.5 Softwood Pitting.- 2.6 Microscopic Studies of Flow in Softwoods.- 2.7 Structure of Hardwoods.- 2.8 Hardwood Pitting.- 2.9 Microscopic Studies of Flow in Hardwoods.- 2.10 Chemical Composition of Normal Wood.- 2.10.1 Cellulose.- 2.10.2 Hemicelluloses.- 2.10.2.1 Introduction.- 2.10.2.2 Softwood Hemicelluloses.- 2.10.2.3 Hardwood Hemicelluloses.- 2.10.3 Lignins.- 2.11 Chemical Composition of Reaction Wood.- 2.11.1 Introduction.- 2.11.2 Compression Wood.- 2.11.3 Tension Wood.- 2.12 Topochemistry of Wood.- 3 Permeability.- 3.1 Introduction.- 3.2 Darcy's Law.- 3.3 Kinds of Flow.- 3.4 Specific Permeability.- 3.5 Poiseuille's Law of Viscous Flow.- 3.6 Turbulent Flow.- 3.7 Nonlinear Flow Due to Kinetic-Energy Losses at the Entrance of a Short Capillary.- 3.8 Knudsen Diffusion or Slip Flow.- 3.9 Corrections for Short Capillaries.- 3.10 Permeability Models Applicable to Wood.- 3.10.1 Simple Parallel Capillary Model.- 3.10.2 Petty Model for Conductances in Series.- 3.10.3 Comstock Model for Softwoods.- 3.10.4 Characterization of Wood Structure from Permeability Measurements.- 3.11 Measurement of Liquid Permeability.- 3.12 Measurement of Gas Permeability.- 3.13 The Effect of Drying on Wood Permeability.- 3.14 Treatments to Increase Permeability.- 3.15 The Effect of Moisture Content on Permeability.- 3.16 The Influence of Specimen Length on Permeability.- 3.17 Permeability of the Cell Wall.- 3.18 Zones of Widely Differing Permeabilities in Wood.- 3.19 General Permeability Variation with Species.- 4 Capillary and Water Potential.- 4.1 Surface Tension.- 4.2 Capillary Tension and Pressure.- 4.3 Mercury Porosimetry.- 4.4 Influence of Capillary Forces on the Pressure Impregnation of Woods with Liquids.- 4.5 Collapse in Wood.- 4.6 Pit Aspiration.- 4.7 The Relationship Between Water Potential and Moisture Movement.- 4.8 Notes on Water Potential. Equilibrium Moisture Content, and Fiber Saturation Point of Wood.- 5 Thermal Conductivity.- 5.1 Fourier's Law.- 5.2 Empirical Equations for Thermal Conductivity.- 5.3 Conductivity Model.- 5.4 Resistance and Resistivity Conductance and Conductivity.- 5.5 Derivation of Theoretical Transverse Conductivity Equation.- 5.6 Derivation of Theoretical Longitudinal Conductivity Equation.- 5.7 R and U Values Convection and Radiation.- 5.8 Application to Electrical Resistivity Calculations.- 5.9 Application to Dielectric Constant Calculations.- 6 Steady-State Moisture Movement.- 6.1 Fick's First Law Under Isothermal Conditions.- 6.2 Bound-Water Diffusion Coefficient of Cell-Wall Substance.- 6.3 The Combined Effect of Moisture Content and Temperature on the Diffusion Coefficient of Cell-Wall Substance.- 6.4 Water-Vapor Diffusion Coefficient of Air in the Lumens.- 6.5 The Transverse Moisture Diffusion Model.- 6.6 The Importance of Pit Pairs in Water-Vapor Diffusion.- 6.7 Longitudinal Moisture Diffusion Model.- 6.8 Nonisothermal Moisture Movement.- 6.9 Measurement of Diffusion Coefficients by Steady-State Method.- 7 Unsteady-State Transport.- 7.1 Derivation of Unsteady-State Equations for Heat and Moisture Flow.- 7.2 Derivation of Unsteady-State Equations for Gaseous Flow in Parallel-Sided Bodies.- 7.3 Graphical and Analytical Solutions of Diffusion-Differential Equations with Constant Coefficients.- 7.3.1 Solutions of Equations for Parallel-Sided Bodies.- 7.3.2 Solutions of Equations for Cylinders.- 7.3.3 Simultaneous Diffusion in Different Flow Directions.- 7.3.4 Significance of Flow in Different Directions.- 7.3.5 Special Considerations Relating to the Heating of Wood.- 7.4 Relative Values of Diffusion Coefficients.- 7.5 Retention.- 7.6 Unsteady-State Transport of Liquids.- 7.6.1 Parallel-Sided Bodies, Permeability Assumed Constant with Length.- 7.6.2 Parallel-Sided Bodies with Permeability Decreasing with Length (Bramhall Model).- 7.6.3 Cylindrical Specimens.- 7.6.4 Square and Rectangular Specimens.- 7.7 Unsteady-State Transport of Moisture Under Noniso-thermal Conditions.- 7.8 Heat Transfer Through Massive Walls.- References.- Symbols and Abbreviations.

Transport processes in wood

Wood is formed in an essentially water-saturated environment in the living tree, and the cell wall remains in this state until the water flow from the roots is interrupted, such as by felling the tree. The wood then begins to lose most of its moisture by drying, resulting in changes in most of its physical properties. These changes, and their relationship to the environment to which the wood is subsequently ex posed, are the subject of this book. The text consists of six chapters. The first chapter discusses cer tain empirical relationships between wood and water, methods of measuring wood moisture content, factors which affect its equilib rium moisture content, and the effect of moisture content on wood strength. The second chapter treats the thermodynamics of moisture sorption by wood, inc1uding enthalpy, entropy, and free energy changes. The third chapter discusses some of the theories which have been proposed to explain the sorption isotherms for hygroscopic ma terials such as wood. Chapter 4 considers hygroexpansion or the shrinking and swelling of wood associated with moisture change. Chapter 5 is concerned with how moisture moves through the cell wall of wood in response to both moisture and temperature gradients. The sixth and final chapter discusses the theoretical and practical aspects of the electrical resistance and dielectric properties of wood, in c1uding the principles involved in their application in electrical moisture meters."

Wood-water relations

Lignification and lignin topochemistry - an ultrastructural view.

Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignification and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture and composition. Hormones mediate some of the variability observed and control the process of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants with modified auxin distribution, suggests that auxin provides positional information for the exit of cells from the meristem and probably also for the duration of cell expansion. Poplar sequencing projects have provided access to genes involved in cell wall formation. Genes involved in the biosynthesis of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modify pectins in the cambial region. Specific expression patterns have also been found for expansins, xyloglucan endotransglycosylases and cellulose synthases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the monolignol biosynthetic pathway and by localizing the expression of various enzymes, new insight into the lignin biosynthesis in planta has been gained.

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Unravelling cell wall formation in the woody dicot stem.

The term microfibril angle (MFA) in wood science refers to the angle between the direction of the helical windings of cellulose microfibrils in the secondary cell wall of fibres and tracheids and the long axis of cell. Technologically, it is usually applied to the orientation of cellulose microfibrils in the S2 layer that makes up the greatest proportion of the wall thickness, since it is this which most affects the physical properties of wood. This review describes the organisation of the cellulose component of the secondary wall of fibres and tracheids and the various methods that have been used for the measurement of MFA. It considers the variation of MFA within the tree and the biological reason for the large differences found between juvenile (or core) wood and mature (or outer) wood. The ability of the tree to vary MFA in response to environmental stress, particularly in reaction wood, is also described. Differences in MFA have a profound effect on the properties of wood, in particular its stiffness. The large MFA in juvenile wood confers low stiffness and gives the sapling the flexibility it needs to survive high winds without breaking. It also means, however, that timber containing a high proportion of juvenile wood is unsuitable for use as high-grade structural timber. This fact has taken on increasing importance in view of the trend in forestry towards short rotation cropping of fast grown species. These trees at harvest may contain 50% or more of timber with low stiffness and therefore, low economic value. Although they are presently grown mainly for pulp, pressure for increased timber production means that ways will be sought to improve the quality of their timber by reducing juvenile wood MFA. The mechanism by which the orientation of microfibril deposition is controlled is still a matter of debate. However, the application of molecular techniques is likely to enable modification of this process. The extent to which these techniques should be used to improve timber quality by reducing MFA in juvenile wood is, however, uncertain, since care must be taken to avoid compromising the safety of the tree.

Cellulose microfibril angle in the cell wall of wood fibres

On the Longi tudinal Permeabi l i ty of Green Sapwood of Abies alba Miller and Piceaabies Karst. to Organic Solvents Summary Different organic solvents were passed through green cylindric samples of sapwood of Abies alba Miller and Picea abies Karst, at a pressure equal to 5 cm water column. By this means the factors influencing the rate of flow could be determined. 1. The rate of flow of an organic solvent is essentially dependent on its viscosity. 2. Although viscosity of the solvent influences the rate of flow especially at the onset of filtration, a high surface tension of the solvent can cause a continuous decrease of the rate of flow with the progress of the experiment. On the contrary a low surface tension effects only a small decrease in the rate of flow. 3. Hydrophobie solvents cannot be filtrated through untreated green sapwood even under application of a higher pressure. 4. For the solvents used here, no influence of the molecular size on the rate of flow is detectable.

Per Henrik Norberg

Papers

Physical and Chemical Properties of the Gelatinous Layer in Tension Wood Fibres of Aspen (Populus tremula L.)