20. How can trees suck the water through the xylem tubes so high, to several meters, if 10 meters is the maximum attainable height for an incompressible liquid to rise a capillary-like tube under standard pressure conditions?
Why doesn't the water boil as it reaches the top, owing to the vacuum that would form at the top of the tube? Trees suck water high up by creating negative pressures, owing to transpiration. The pressure difference is enough for the water to reach high up through the xylem tubes. Moreover, the water doesn't boil because the xylem tubes are devoid of any activating agent (say, an air bubble) that would provide the necessary latent energy for the phase conversion to occur.
