sentences:  During the early Devonian, maximum tracheid diameter increased with time, but may have plateaued in the zosterophylls by mid-Devonian.Overall transport rate also depends on the overall cross-sectional area of the xylem bundle itself, and some mid-Devonian plants, such as the Trimerophytes, had much larger steles than their early ancestors.While wider tracheids provided higher rates of water transport, they increased the risk of cavitation, the formation of air bubbles resulting from the breakage of the water column under tension.