Which change in the secondary xylem produces late wood in the fall season?

1. Annual rings arise from the seasonal activity of the vascular cambium. 2. In the spring growing season the secondary xylem cells have a large internal diameter and primary walls that are not extensively thickened, which gives early wood. 3. In the fall season the secondary xylem develops thickened cell walls, and this late wood is denser than early wood. 4. So the thickened walls of autumn wood are the correct change. 5. Wide diameters and unthickened walls describe spring wood, which is the trap set by reversing the seasons. 6. The alternation is driven largely by a seasonal fall in vessel element number and a seasonal rise in tracheid number, not by wood becoming hollow. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.2 Stems_