The trunk in section
Outside is the bark, under it a thin living layer, the cambium. Every year the cambium adds new wood inwards and new inner bark outwards. The pale outer wood, the sapwood, still carries water. The dark core in many species, the heartwood, no longer conducts and just holds the tree up.
The cambium is a lateral meristem: its divisions produce xylem inwards and phloem (inner bark) outwards. Sapwood contains living parenchyma cells; when heartwood forms they die, vessels and pits become blocked and extractives accumulate, so heartwood is often darker and more decay resistant. Wood anatomy is studied on three planes: transverse (cross), radial and tangential.
Softwoods: almost all tracheids
In pine, spruce and larch almost the whole wood is made of one cell type, tracheids. They are long hollow tubes with tapered ends that both carry water and hold the tree. Tracheids talk to each other through tiny valves, bordered pits.
Tracheids make up more than 90% of softwood volume. Earlywood tracheids are thin walled with wide lumina; latewood tracheids are thick walled and radially flattened. Softwoods have no vessels. Rays are narrow, usually one cell wide, built of parenchyma cells; in pine, spruce and larch they also contain ray tracheids. Pine, spruce and larch have resin canals lined with epithelial cells that secrete resin.
Hardwoods: vessels and fibres
In birch, oak and maple the jobs are split. Wide pipes, vessels, move the water. Narrow thick walled fibres give strength. On the end grain the vessels show up as pores, which is why hardwoods are called porous woods and softwoods non-porous.
Vessels are built of short vessel elements with perforated end walls. Mechanical tissue is formed by libriform fibres and fibre tracheids, storage by parenchyma. By vessel arrangement, ring-porous woods (oak, ash) concentrate large vessels in the earlywood, while in diffuse-porous woods (birch, maple, poplar) vessels are spread evenly across the ring.
The cell wall
Each cell wall is layered like plywood. Cells are glued together by the middle lamella, rich in lignin. Inside is a thin primary wall and then a thick secondary wall in three layers: S1, S2 and S3. The thickest layer, S2, sets the properties of wood.
The middle lamella (ML) and primary wall (P) are often treated together as the compound middle lamella. In the secondary wall the cellulose microfibrils run at different angles: S1 at roughly 50-70° to the cell axis, S2 at 10-30°, S3 at 60-90°. Lignin concentration is highest in the middle lamella and cell corners, yet most of the lignin mass sits in the secondary wall because it is so much thicker.
Reaction wood
When a trunk leans, the tree straightens itself with special wood. Softwoods push the stem up from below, hardwoods pull it from above. This wood has different chemistry and behaves differently in processing.
Softwoods form compression wood on the lower side of a leaning stem: rounded tracheids with intercellular spaces, usually no S3 layer and clearly more lignin than normal wood. Hardwoods form tension wood on the upper side, with a gelatinous G-layer made almost entirely of cellulose and poor in lignin.
Key facts
- tracheids in softwood
- >90% of volume
- tracheid length
- typically 2-4 mm
- wall layers
- ML, P, S1, S2, S3
- microfibril angle in S2
- 10-30°
Sources for this chapter
- Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.
- Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.
- Fengel D., Wegener G. Wood: Chemistry, Ultrastructure, Reactions. Berlin; New York: De Gruyter, 1984.
- Ross R.J. (ed.) Wood Handbook: Wood as an Engineering Material. General Technical Report FPL-GTR-282. Madison, WI: USDA Forest Service, Forest Products Laboratory, 2021.