36 Drying of Wood

(a) Norway spruce (Picea abies) (b) Pedunculate oak (Quercus rubra) (c) European beech (Fagus sylvatica)FIGURE 36.5 Typical anatomical patterns encountered in temperate species: (a) softwood, (b) ring-porous species, and (c)diffuse-porous species. The height of these images represents about 2 mm. (Microphotographs: J.C. Mosnier, LERMAB–ENGREF.)After some years, an intense biological activity ofthese parenchyma cells gives rise to heartwood formation.Metabolites are deposited in the heartwoodand the tree uses the heartwood as a place to storewaste products that are collectively known as extractives.These include resins, gums, oils, and tanninsthat stop up the vessels (or the tracheids in softwood)and clog the wood. In heartwood, all the cells aredead and inactive; they do not function in eitherwater conduction or food storage. The heartwood isoften darker, slightly denser, and more durable (resistanceto fungi or insect attack) than the sapwoodand plays an important role in supporting the tree.However, numerous species do not have dark heartwood(e.g., spruce, fir, and beech) and no correlationexists between heartwood color and durability.In some species, such as certain oaks, the vesselsbecome plugged with the development of cellularmembranes known as tyloses, which enter the vesselsfrom adjacent parenchyma cells (Figure 36.6).36.1.2 CHEMICAL C OMPOSITIONWood is a typical organic material made up of threemain elements: carbon, oxygen, and hydrogen. Becausevery small variations between different wood speciesare observed, the numbers depicted in Table 36.1 arebroadly general but can be higher for some tropicalspecies. Nitrogen as well as some additional inorganicelements (sodium, potassium, calcium, magnesium, andsilicon) are also essential compounds, which are mostlyinvolved in the metabolism of living cells during woodFibersRay cellsTyloses invesselsRTTLFIGURE 36.6 Example of tyloses development in the heartwood of Pedunculate oak (Quercus rubra L.). (ESEM photographs:Patrick Perré and Riad Bakour, LERMAB–ENGREF.)ß 2006 by Taylor & Francis Group, LLC.