1 Introduction.- 1.1 Prehistoric and Ancient Use.- 1.2 Changing Uses of Forests.- 2 Definitions and Descriptions.- 2.1 Introduction.- 2.2 Sapwood.- 2.2.1 Definitions.- 2.2.1.1 Sapwood.- 2.2.1.2 Included Sapwood.- 2.2.2 Area of Sapwood in Log Cross-Sections.- 2.2.3 Variation of Area of Sapwood.- 2.2.3.1 In a Species.- 2.2.3.2 In Different Species.- 2.2.3.3 Due to Age of Tree.- 2.2.3.4 Due to Rate of Growth.- 2.2.3.5 Due to Environment.- 2.2.4 Sapwood Contents.- 2.2.5 Discolored Sapwood.- 2.3 Intermediate Wood.- 2.3.1 Description.- 2.3.2 Occurrence.- 2.4 Transition Zone.- 2.4.1 Definition.- 2.4.2 Description.- 2.4.3 Occurrence.- 2.4.3.1 Heartwood Boundary Stain.- 2.5 Heartwood.- 2.5.1 Definition.- 2.5.2 Description.- 2.5.2.1 Appearance.- 2.5.2.2 Level of Maximum Area.- 2.5.2.3 Regular Heartwoods.- 2.5.2.4 Irregular Heartwoods.- 2.5.2.5 Variations in Appearance.- 2.6 Tree Exudates and Extracellular Materials.- 2.6.1 Definitions.- 2.6.1.1 Intercellular Secretory Spaces, Cavities, and Shakes.- 2.6.1.2 Intercellular Canals.- 2.6.1.3 Pitch Tubules and Stones.- 2.6.1.4 Pockets, Veins, and Streaks.- 2.6.2 Types of Exudate or Deposit.- 2.6.2.1 Resin.- 2.6.2.2 Gum.- 2.6.2.3 Kino.- 2.6.2.4 Latex.- 2.6.2.5 Manna.- 2.6.2.6 Amber.- 2.6.2.7 Balsam.- 2.6.2.8 Maple Sugar.- 2.6.2.9 Crystalline Compounds.- 2.6.3 Rate of Formation.- 3 Historical Aspects.- 3.1 The Use of Durable Woods.- 3.2 Exudates.- 3.3 Varnishes and Lacquers.- 3.4 Gums.- 3.5 Tannins.- 3.6 Dyes.- 3.7 Perfumes.- 3.8 Rubber.- 3.9 Medicines.- 3.10 Lessons from History.- 4 Influence of Forestry Aspects.- 4.1 Variation of Heartwood Volume.- 4.1.1 Heritability.- 4.1.2 Effect of Growth Rate and Crown Size.- 4.1.3 Influence of Environment.- 4.1.4 Influence of Injury and Health.- 4.2 Formation of Exudates.- 4.2.1 From Bark and Wood.- 4.2.2 From Wood.- 4.2.3 Addition of Stimulants.- 5 Chemical Features.- 5.1 Water and Gases.- 5.2 Inorganic Compounds.- 5.3 Storage Substances and Intermediates.- 5.4 Nitrogenous Compounds.- 5.5 Ethylene.- 5.6 Type of Extractives.- 5.6.1 Galactans and Cyclitols.- 5.6.2 Terpenoids.- 5.6.3 Fatty Acids and Related Compounds.- 5.6.4 Phenolic Compounds.- 5.6.4.1 Simple Phenols and Phenolic Acids.- 5.6.4.2 Lignans.- 5.6.4.3 Stilbenoids.- 5.6.4.4 Flavonoids.- 5.6.4.5 Quinones.- 5.6.4.6 Polymerized Polyphenols.- 5.6.4.7 In Different Tissues.- 5.7 Amount of Extractives.- 5.7.1 Position of Sample in the Tree.- 5.7.2 Effects of Rate of Growth 1ll.- 5.7.3 Effect of Site.- 5.7.4 Genetic Differences.- 5.7.5 Crystals.- 5.8 Reagents for Heartwood Detection.- 5.9 Exudates.- 5.9.1 Resin.- 5.9.2 Gum.- 5.9.3 Kino.- 5.9.4 Latex.- 5.9.5 Manna.- 6 Biological Features.- 6.1 Sapwood.- 6.1.1 Wood Tissues.- 6.1.2 Lumen Volume.- 6.1.3 Deposition of Extractives on Wall Surfaces.- 6.1.4 Impregnation of Cell Walls.- 6.1.5 Parenchyma.- 6.1.5.1 Volume.- 6.1.5.2 Cell Wall.- 6.1.5.3 Cytology.- 6.1.6 Respiration and Enzymes Activity.- 6.2 Transition Zone.- 6.2.1 Seasonal Variations.- 6.2.2 Appearance.- 6.2.3 Water Content.- 6.2.4 Pit Aspiration and Tylosis Formation.- 6.2.5 Cytology of Parenchyma Cells.- 6.2.6 Respiration.- 6.2.7 Enzyme Activity.- 6.2.8 Formation of Extractives.- 6.3 Heartwood.- 6.3.1 Seasonal Formation.- 6.3.2 Appearance.- 6.3.3 Respiration and Enzyme Activity.- 6.3.4 Location of Extractives.- 6.4 Wound Wood and Chemically Affected Wood.- 6.4.1 Wound Wood.- 6.4.2 Paraquat-Treated Woods.- 6.4.2.1 Biochemical Changes Due to Paraquat.- 6.4.3 Ethylene-Treated Wood.- 6.4.4 Knots.- 6.5 Exudates.- 6.5.1 General.- 6.5.2 Resin Formation.- 6.5.2.1 Anatomy of Pockets.- 6.5.3 Gum Formation.- 6.5.4 Kino Formation.- 6.5.4.1 Anatomy of Veins and Pockets.- 6.5.4.2 Chemistry of Kino Formation.- 6.5.5 Rubber Tapping.- 7 Function, Formation, and Control of Heartwood and Extractives.- 7.1 Function and Volume of Sapwood.- 7.1.1 Function.- 7.1.2 Volume.- 7.2 Types and Formation of Heartwood.- 7.2.1 Introduction.- 7.2.2 Types of Heartwood.- 7.2.2.1 Regular Heartwoods.- 7.2.2.2 Other Types.- 7.2.3 Conclusions.- 7.3 Features of Heartwood and Woundwood.- 7.3.1 Some Theories of Heartwood Formation.- 7.3.1.1 Natural Causes.- 7.3.1.2 Accumulation of Gas and Control of Water Content.- 7.3.1.3 Initiation by Fungi and Hormones.- 7.3.2 Anatomical Changes.- 7.3.3 Occurrence of Extractives.- 7.4 The Transition Zone and its Formation.- 7.5 Function of Extractives and Exudates.- 7.6 Formation of Exudates and Extractives.- 7.6.1 Differences in Composition.- 7.6.2 Site of Formation.- 7.6.2.1 Exudates.- 7.6.2.2 Extractives.- 7.6.3 Amounts.- 7.6.4 Type of Extractives in Tissues.- 7.7 Initiation of Formation of Heartwood, Extractives, and Exudates.- 7.7.1 Initiation by Displacement of Water.- 7.7.2 Initiation by Changes in Ethylene Levels and in Hormonal Balance.- 7.8 Factors Controlling the Nature of Extractives and Exudates.- 7.9 Activities at Cellular Levels.- 7.10 Conclusions.- References.

Heartwood and Tree Exudates

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.

Form and function of arabinogalactans and arabinogalactan-proteins

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

Pronounced tension wood from four North-American hardwood species has been examined by light and electron microscopy. Delignified fibers were also studied. The gelatinous layer was in all cases loosely attached to S2 but varied considerably in thickness within each species and was in one case terminated towards the lumen by a layer resembling S3. A terminal lamella was not observed. After considering both earlier evidence and the present results, it was concluded that the gelatinous layer has neither a honeycomb nor a homogeneous texture, as has been suggested, but that it consists of concentric lamellae of cellulose microfibrils. In the absence of hemicelluloses and lignin, the microfibrils are probably bound together less firmly than they are in other cell wall layers. The gelatinous layer is more readily separated from the remainder of the cell wall by mechanical forces than by chemical reagents.

A contribution to the ultrastructure of tension wood fibers

Studies on Larch Arabinogalactan. I. The Distribution of Arabinogalactan in Larch Wood

The distribution of lignin has been studied in tracheids and ray cells of normal and compression wood of tamarack [Larix laricina (Du Roi) K. Koch]. The three layers in the secondary wall of normal wood tracheids are lignified to approximately the same extent, and previous evidence that the S
3 layer should contain a higher proportion of lignin than the other regions has not been confirmed. The lignin follows closely the orientation of the cellulose microfibrils in all three layers. Compared to the tracheids, the ray cells contain a denser network of lignin in their secondary wall. Only a small proportion of the total lignin in compression wood tracheids is present in the compound middle lamella. The thick S
1 layer is only slightly lignified; the orientation of the lignin in this region is that of the transversely oriented, lamellated microfibrils. The outer portion of S
2 consists largely of lignin but also contains lamellae of cellulose microfibrils which probably have the same helical orientation as the microfibrils in the inner part of S
2. The latter region, which contains the helical cavities, consists of lamellae of cellulose microfibrils which are uniformly encrusted with lignin. The ray cells in compression wood appear to be lignified to the same extent as in normal wood. Transverse sections of the cells reveal a lateral orientation of the lignin. The orientation of the cellulose microfibrils in the S
2 layer of the first-formed springwood tracheids of compression wood is the same as in the cells which are formed later. It is suggested that for ease of reference, the outer, lignin-rich layer in compression wood tracheids be referred to as the S
2(L) layer.

Studies on compression wood—part VII: Distribution of lignin in normal and compression wood of tamarack [Larix laricina (Du Roi) K. Koch]

The distribution of lignin in normal and tension wood of four hardwood species has been studied by examination in the electron microscope of the lignin skeletons remaining after removal of the polysaccharides with hydrofluoric acid. In normal wood fibers, the S1 had a higher lignin concentration than the S2 layer, which was not as highly lignified as in conifer tracheids. Vessels had a high concentration of lignin in both normal and tension wood, while the extent of lignification of the parenchyma was variable.

W. A. Côté

Papers

A contribution to the ultrastructure of tension wood fibers

Studies on Larch Arabinogalactan. I. The Distribution of Arabinogalactan in Larch Wood

Studies on compression wood—part VII: Distribution of lignin in normal and compression wood of tamarack [Larix laricina (Du Roi) K. Koch]

Distribution of lignin in normal and tension wood

Studies on Compression Wood. V. Nature of the Compression Wood Formed in the Early Springwood of Conifers