How water climbs without a pump

Giant coast redwoods move water from their roots to their leaves 358 feet (109 m) up, a height that may be close to the most xylem can manage (Ha et al., n.d.). No pump. No active machinery. Root pressure can push water up about 60 feet (18 m) at most, and only at night or in cloudy weather, and only in some plants; while the sun is out, the lifting is done by a pull from the leaves (Michaels et al., 2022). The mechanism is cohesion-tension, and it's one of the things we find most remarkable in biology.

Water molecules stick to each other (cohesion, from hydrogen bonds) and to the xylem walls (adhesion). When water evaporates from a leaf at the top, it tugs the molecule just below it. That one, stuck to all its neighbors and to the walls, gets pulled up, pulling the next, and the next. From the soil to the leaf, the water forms one unbroken column that behaves like a rope pulled tight (Venturas et al., 2017). Pull at the top and the bottom rises: evaporation at the leaf puts the water under tension, and that tension draws water up from the roots (Clark et al., 2018). The evaporation doing the pulling has a name, transpiration, and Lesson 8 is about its budget.

That tension is why xylem walls are stiffened with lignin (the tough compound in wood) and a thick extra wall layer: without them the suction inside would make the tubes cave in (Venturas et al., 2017, summarising Hacke et al., 2001). (Cutting a stem whose water is under tension can let air into the open vessels, and imaging suggests this happens even when the cut is made underwater, the standard method (Venturas et al., 2017, summarising Torres-Ruiz et al., 2015). In one study, chrysanthemum flowers were left to dry a little and then recut in air, which the authors say made the stems take in air at once (van Ieperen et al., 2002). Stem pieces for that study's flow tests were prepared underwater to stop air getting in at the start (van Ieperen et al., 2002). That's our reasoning for recutting flower stems underwater.)

Fig. 6. Evaporation at the leaves creates tension that pulls the whole water column up through the xylem. Molecules cohere to each other and adhere to the walls. No pump required.