36 Drying of Wood

Clearwood formedafter pruningDeath of thebranchNonadhesive partof the knotAdhesive part ofthe knotPithFIGURE 36.3 The existence of knots in wood comes from the interaction between primary growth, responsible for thebranch formation, and secondary growth, responsible for thickening of stem and branches. (Photograph: Scots pine (Pinussylvestris), LERMAB–ENGREF.)weeks; the last development stage, namely lignification,induces the death of these cells. Only parenchymacells can live for years and they are responsible for thedevelopment of heartwood.In most species in temperate climates, the differencebetween woods that are formed early in a growing season(earlywood) and that formed later (latewood) issufficient to produce concentric contours in a crosssection (Figure 36.2). These rings are known as growthrings. Each increment in size in the branch or trunkdiameter can be observed in these growth rings thatremain unchanged once formed. Provided no falserings exist (due to an interruption of the growth indiameter by drought or defoliation by insects), the ageat any cross section of the trunk may be determined bycounting these rings. Obviously, this simple rule doesnot apply for tropical species for which growth may bepractically continuous throughout the year and no welldefinedgrowth rings are formed, or for which growthringsare the result of the individual rhythmofeachtree.Wood cells are of various sizes and shapes. Theyare cemented together to form the structural woodmaterial. The majority of wood cells are elongatedand pointed at the ends. The types and dimensionsof wood cells depend strongly on the species.In softwoods, woods formed by cone-bearing trees(e.g., fir, pine, and spruce) with naked seeds, the xylemcontains mainly tracheids (90%). Tracheids are considerablyelongated cells (around 40 mm in diameterand between 2 and 8 mm in length), which ensure bothsap flow, by means of numerous bordered pits situatedon the radial cell walls, and mechanical strength.In softwoods, the earlywood is characterized by cellswith large radial diameters and thin walls, and hencerelatively large cavities. Latewood cells have a muchsmaller radial diameter and thicker walls, which resultin much smaller cavities (Figure 36.4). In addition,some softwoods have resin canals. Parenchyma cellssurround these canals and actively secret resin into thecanals, and ultimately into the heartwood.Hardwood is the common name for the wood ofspecies whose seeds are enclosed in ovaries. Thesespecies are more advanced than softwoods in termsof biological evolution; consequently, they produce amore sophisticated anatomical pattern, with cellsmuch more adapted to meet specific requirements inrelation to water transport, food storage, and mechanicalsupport:. Fibersareusuallyrelativelythick-walled,sparselypitted, and about 1 mm in length. Different fibercells exist, but the tracheid fibers having borderedpits are generally the most abundant. Althoughfibers may have a certain role in sap conduction,theybasicallyfunctionasthemechanicalsupport,making the wood usually stronger, denser, andmore durable than softwoods.. Vessels of relatively large diameter are alsoknown as pores. These cells form the main conduitsfor sap flow. A vessel is built up by severalvessel elements, with more or less open endplates, aligned in the longitudinal direction.ß 2006 by Taylor & Francis Group, LLC.