Trees lift water a hundred metres using pipes made of dead cells
The plumbing that carries water up a tree is built from cells that have already died. Xylem conduits are empty tubes, and the pull comes from leaves: as water evaporates, tiny curved menisci in cell walls create tension strong enough to haul sap up to about a hundred metres.
Xylem is one of two transport tissues in vascular plants, partnered with phloem inside the vascular bundles. Its main job is moving water and dissolved minerals up from the roots to stems and leaves. The name comes from the Greek word for wood, and wood is indeed the most familiar xylem, although the tissue runs through the whole plant. Carl Nägeli coined the term in 1858. Its signature cells are tracheary elements: long tracheids, and shorter vessel elements joined end to end into continuous vessels.
Because those cells are dead at maturity, the transport is passive. Several forces drive it. At the top, evaporation from leaf cells draws water back into microscopic pores, and surface tension across the resulting menisci puts the whole water column under tension, dragging more up from the soil. Adhesion between water and conduit walls supports this capillary rise. From below, roots with a high solute concentration can draw in water by osmosis and push sap upward, a root pressure that peaks in the morning before stomata open. It varies widely: up to 145 kPa in the riverbank grape Vitis riparia, close to zero in oriental bittersweet.
Pulling water under tension is risky. The vessels must be very narrow, or cavitation would snap the column. In dry soil, gas can come out of solution and form an embolism, an air bubble that would spread through neighbouring cells if not for bordered pits, which carry a plug called a torus that seals the gap. Plants can even refill blocked xylem afterwards. This difficulty of lifting water is thought to set an upper limit on how tall trees can grow.
Secondary xylem, laid down by the vascular cambium, is what the timber trade sells. All of the roughly 600 conifer species produce it, and their wood is marketed as softwood. Among some 250,000 angiosperm species it is rare in monocots, but the non-monocot trees supply hardwood.
Source: Xylem
