A hard university course made clear. Dive into wood with Russian textbooks and world research as your guide.
A tree grows from the outside, not the inside. A thin layer under the bark, the cambium, adds a new layer of wood every year. That is where annual rings come from.
The cambium is a lateral meristem between phloem and xylem. Outer rings form the sapwood that conducts water. In many species the inner rings become heartwood: the living parenchyma cells die and extractives accumulate there.
In spring the tree builds wide cells with thin walls to move water fast. In summer the walls get thicker and the openings narrower. That gives the light and dark bands of a ring.
In softwoods, earlywood tracheids are thin walled with wide lumina and mostly conduct water. Latewood tracheids are thick walled, flattened radially and carry most of the mechanical load, so latewood is markedly denser.
Pine and spruce wood is built almost entirely from long hollow cells called tracheids. They are dead: only the walls remain, like a honeycomb made of tubes.
Tracheids make up more than 90% of softwood volume and are typically 2-4 mm long. Neighbouring cells connect through bordered pits. In hardwoods, vessels conduct water while libriform fibres give strength.
The wall is layered like plywood. In each layer the cellulose threads run at their own angle. The thickest layer, S2, runs almost along the cell, and that is what makes wood strong in tension.
The middle lamella (ML) glues cells together and is rich in lignin. Then come the primary wall P and the secondary wall: S1 with microfibrils at roughly 50-70° to the cell axis, S2 at 10-30° forming the bulk of the wall, and S3 at 60-90°.
Zoom further and the wall looks like reinforced concrete. The rebar is thin cellulose threads, the concrete is lignin, and hemicelluloses bind the two together.
A cellulose microfibril about 3 nm thick holds roughly 18-24 parallel chains with more and less ordered regions. Hemicelluloses adsorb onto its surface; lignin fills the space between and is covalently linked to hemicelluloses in lignin-carbohydrate complexes.
Cellulose is a chain of thousands of glucose units. Each unit is flipped relative to the previous one, so the chain is straight and flat, and neighbouring chains stick together with hydrogen bonds.
Cellulose is a β-(1→4)-D-glucan with cellobiose as the 1.03 nm repeating unit. The degree of polymerisation of native wood cellulose is around 10 000. Hydrogen bonds, together with hydrophobic stacking of the chains, account for its crystallinity and insolubility in water.
Underneath it all there are just three elements. About half the mass of wood is carbon, over 40% is oxygen and about 6% is hydrogen. Nitrogen and minerals are fractions of a percent.
The elemental composition of temperate woods barely depends on species: about 50% C, 6% H and 44% O; nitrogen usually 0.1% or less, ash usually below 0.5% (rarely up to 1%) of dry mass.
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Two thirds of dry wood are carbohydrates, a quarter is lignin. Softwoods carry more lignin and mannans, hardwoods more xylan.
Sjöström's data for Scots pine and silver birch, % of dry wood. Softwood lignin is almost pure guaiacyl type; hardwood lignin mixes guaiacyl and syringyl units.
One square equals 1% of dry wood mass. Sjöström 1993.
Long straight chains of glucose packed side by side. They form the strong threads of the cell wall. Cotton is almost pure cellulose, and most paper is made mainly of it.
Linear homopolysaccharide of β-D-glucopyranose units joined by (1→4) glycosidic bonds. Chains aggregate into microfibrils of cellulose I; in wood the Iβ form dominates. Degree of polymerisation around 10 000.
Shorter, branched chains made of several sugars. They coat the cellulose threads and bind them to lignin, like glue between rebar and concrete.
Heteropolysaccharides with DP around 100-200. Softwoods: mainly galactoglucomannans plus arabinoglucuronoxylan. Hardwoods: mainly O-acetyl-4-O-methylglucuronoxylan with a small amount of glucomannan.
A three-dimensional network built from ring-shaped units. It fills the gaps in the cell wall, makes it rigid and less permeable to water, and makes it harder for microbes and fungi to digest the wall.
Aromatic polymer formed by radical coupling of three monolignols: p-coumaryl (H), coniferyl (G) and sinapyl (S) alcohols. The β-O-4 aryl ether is the most frequent linkage: roughly half of all bonds in softwood lignin and about 60% in hardwood lignin.
Freshly cut wood is full of water. First the free water leaves the cell cavities and nothing changes in size. Only when the walls start to dry does the wood shrink and crack.
Below the fibre saturation point (about 30% moisture content) water is bound to hydroxyl groups of the cell wall polymers, and losing it causes shrinkage. Tangential shrinkage is roughly twice the radial one, which is why logs check radially.
Synthetic polymers are built the same way as natural ones: small molecules join into long chains. Plywood and particleboard are held together by such resins.
Chain (addition) polymerisation joins monomers without by-products, for example in polyvinyl acetate glue. Polycondensation releases a small molecule, usually water, as the chains grow: phenol-formaldehyde and urea-formaldehyde resins cure this way.
Read the chapter →Every number on the site has a source. Russian textbooks give the course structure; international monographs and papers give the details.