Xylem

Xylem is the vascular tissue that carries water and dissolved minerals from the roots to the rest of a plant, pulled upward under tension by evaporation from the leaves. Together with phloem, which distributes sugars, xylem forms the plumbing of vascular plants, and in trees the accumulated secondary xylem is what we call wood.

Xylem is also one of the great engineering achievements of the biosphere: a network of dead, hollow cells that holds a continuous column of water under negative pressure from the soil to leaves more than 100 meters above the ground.

Xylem is a complex tissue built from several cell types:

• Tracheids are long, narrow cells with tapered ends, found in all vascular plants. Water passes from one tracheid to the next through thin regions of the wall called pits. Conifers rely almost entirely on tracheids.
• Vessel elements are wider, shorter cells joined end to end into continuous tubes (vessels) through perforated end walls. Vessels occur in most flowering plants and evolved independently in a few other lineages, such as the gnetophytes and some ferns. Their width makes them far more efficient conductors than tracheids.
• Fibers add mechanical strength, and living parenchyma cells store starch and help repair and refill the system.

Conducting cells are dead at maturity. Before they die, they build thick secondary walls reinforced with lignin, which keeps them from collapsing under tension. The cells then dismantle their own contents, leaving empty pipes.

Water moves up the xylem by a passive mechanism first proposed by Henry Dixon and John Joly in 1894:

1. Evaporation: Water evaporates from the wet cell walls inside leaves and escapes through the stomata during transpiration.
2. Tension: The evaporating surfaces form tiny curved menisci in the cell-wall pores, which pull on the water behind them, so the water in the xylem is under negative pressure, typically several tenths of a megapascal to a few megapascals.
3. Cohesion: Because water molecules cling to one another through hydrogen bonds, the pull is transmitted down an unbroken column through stems and roots to the soil.

The plant spends no metabolic energy lifting the water; the energy comes from the sun, which drives evaporation. In some small plants, root pressure created by ion pumping in the roots can push water upward at night, producing droplets at leaf margins (guttation), but root pressure cannot lift water to the tops of tall trees.

Water under tension is in a metastable state. If tension becomes too great, air can be pulled through pit membranes into a conduit, where it expands into a gas bubble, an embolism (or cavitation), that blocks flow. Pit membranes act as safety valves, isolating embolized conduits from their neighbors.

There is a trade-off between efficiency and safety: wide vessels conduct water quickly but tend to embolize more readily. During severe drought, widespread embolism can cause hydraulic failure, now recognized as a primary mechanism of drought-driven tree mortality. The xylem's vulnerability to embolism is therefore a key trait for predicting how forests respond to climate change.

In trees and shrubs, a ring of dividing cells called the vascular cambium adds new xylem (inward) and phloem (outward) each year. In seasonal climates, wide spring cells and dense late-season cells form visible growth rings. Their widths and chemistry preserve year-by-year records of temperature and rainfall used in paleoclimatology.

Xylem also helps set the upper limit of tree height. The tallest known living tree, a coast redwood named Hyperion, is about 116 meters tall. Hydraulic studies of redwoods suggest that gravity and the resistance of a very long water path limit maximum height to roughly 120–130 meters, because leaves at the top can barely draw enough water to keep their stomata open.

The water that rises through xylem and leaves through stomata is a major part of continental evapotranspiration, recycling rainfall back into the atmosphere and helping drive the water cycle over forests such as the Amazon. The lignified walls of xylem cells, made largely of cellulose and lignin, store a large share of the carbon held in living vegetation, making wood a key reservoir in the carbon cycle.