sentences:  Small pits in tracheid walls allow water to by-pass a defective tracheid while preventing air bubbles from passing through but at the cost of restricted flow rates.By the Carboniferous, Gymnosperms had developed bordered pits, valve-like structures that allow high-conductivity  pits to seal when one side of a tracheid is depressurized.Tracheids have perforated end walls, which impose a great deal of resistance on water flow, but may have had the advantage of isolating air embolisms caused by cavitation or freezing.