With increased age, the wall may thicken by addition of more cellulose and by the introduction of lignin, a hardening substance. Hardwoods like oak and ash are made up of cells with heavily lignified walls. All of these extra layers constitute the cell’s secondary wall. Cellulose is laid down in microscopic threads called microfibrils; lignin forms deposits on the cellulose surface. Each new layer of wall material, produced by the living cytoplasm, is set in place inside the previously formed layer.
