Cells and cell walls: tracheids, vessels, fibres and rays

Two ways of building a tree

Softwood and hardwood are botanical terms, not statements of hardness. Balsa, a hardwood, has a specific gravity below 0.20, while some tropical hardwoods exceed 1.0 (Wood Handbook, North America). What really differs is the cell structure, visible with a 10x hand lens.

Softwood: tracheids do almost everything

According to the Wood Handbook, tracheids make up over 90 % of a softwood by volume. A tracheid is a long, thin, hollow cell, 1 to 10 mm long and more than 100 times longer than wide. The same cells carry water in the living tree and the load in the finished board.

Spruce, pine, larch and Douglas fir also contain resin canals: voids lined with resin-producing cells. The Czech Timber Atlas (Europe) lists them as absent in fir, but poorly visible on the cross section of spruce and indistinct in larch.

Hardwood: vessels, fibres and rays

Hardwoods share the work between specialised cells. Vessels are the conducting tubes, seen as pores on the end grain. Fibres supply most of the strength. The Wood Handbook gives typical vessel diameters of 50 to 200 micrometres and fibres 0.2 to 1.2 mm long, about half the width of softwood tracheids. The quick field test: pores under a lens mean hardwood, no pores mean softwood.

Hardwoods are classified by how vessel size changes across a ring:

• Ring-porous (oak, ash, elm): large earlywood pores form a distinct band.
• Diffuse-porous (beech, maple, birch, poplar): vessel size is roughly uniform.
• Semi-ring-porous (for example walnut): in between.

The boundaries are gradual, and sources disagree on some species. Romanian researchers do attribute oak's drying difficulty to its ring-porous structure, large rays and tannin-related discolouration (Europe).

Rays: planes of weakness

Rays are ribbons of cells running radially from bark towards the pith. Most hardwood rays are one to five cells wide and under 1 mm high, but oak and hard maple also have rays more than eight cells wide. Two consequences matter to a sawmill:

• Identification. Rays are broad and visible on all faces in oak and beech. In maple, ash and elm the Timber Atlas lists them as visible only on the radial surface. That separates beech from maple, and oak from ash and elm.
• Weakness. The older Wood Handbook states that rays are planes of weakness along which seasoning checks readily develop, and the Dry Kiln Operator's Manual (North America) says honeycomb usually occurs in rays.

Tyloses and permeability

In oak heartwood, tyloses block the earlywood vessels, which LWF Bayern (Germany) gives as the reason for its poor impregnability. In beech, red heartwood has more cells with tyloses, which are reported to slow drying (Slovakia and Poland).

The cell wall in one minute

Each cell wall has layers: the middle lamella, the primary wall and a secondary wall made of S1, S2 and S3 layers around the central cavity (lumen). The S2 layer is the thickest and dominates wall properties. Cellulose threads (microfibrils) wind through it at an angle to the cell axis, the microfibril angle (MFA). The Wood Handbook gives S2 angles of 5 to 30 degrees, and Donaldson's review gives 5 to 20 degrees for mature conifer wood, highest in juvenile wood. A small angle means stiff cells that shrink little along their length; a large angle means lower stiffness and more longitudinal shrinkage (Lesson 5).

Chemically, softwoods carry more lignin (26 to 34 % against 23 to 30 % in the Rowell chapter, North America) and hardwoods much more xylan-type hemicellulose (pentosans 19 to 26 % against 7 to 14 %).

What it means at the saw and in the kiln

• At the saw. Hardwoods with broad rays tear and check more. Resin in spruce, pine and larch builds up on blades and guides, so cleaning routines matter (Teknos, Wood Database, Europe).
• In the kiln. The Dry Kiln Operator's Manual (North America) says softwoods are usually dried by time schedules and hardwoods by moisture-content schedules. No European commercial schedules were found in open sources, so treat any schedule as a starting point for a trial.

Key points

• Softwood is over 90 % tracheids; hardwood has vessels for transport and fibres for strength.
• Pores under a 10x lens mean hardwood; ring-porous, diffuse-porous and semi-ring-porous describe the pore pattern.
• Resin canals occur in spruce, pine, larch and Douglas fir but not in fir, and are hard to see.
• Rays are planes of weakness: broad rays favour checks and honeycomb and help identify oak and beech.
• A large S2 microfibril angle means lower stiffness and more longitudinal shrinkage.