How Do Plants Drink? Water Pulled Upward by Evaporating Leaves

Water evaporates from wet cell surfaces inside the leaf into the air spaces and escapes through pores called stomata. As it leaves, the remaining water is drawn into progressively narrower curved surfaces within the cell wall pores, and the surface tension at those curved interfaces generates a substantial negative pressure. Because water molecules attract each other strongly through hydrogen bonding, and because they also adhere to the walls of the narrow conducting vessels, the entire column from leaf to root behaves as a continuous thread under tension rather than as a series of separate volumes, so pulling at the top moves water at the bottom. The forces involved are large, with tensions in tall trees corresponding to pressures well below zero, and water in that state is metastable, meaning it is being stretched beyond what it would normally tolerate and can fail abruptly. The system uses no metabolic energy for the lifting itself, which is why a cut stem continues to draw water and why the process continues in dead wood, and the energy actually comes from the sun evaporating water at the leaf.

The conducting tissue is built for the job and its details explain several familiar observations:

• •Xylem, the water-conducting tissue, consists of dead cells whose contents have been removed, leaving hollow tubes with reinforced walls that resist collapsing under tension
• •Vessels in flowering plants are wide and efficient, while tracheids in conifers are narrower and less efficient but more resistant to failure, which is part of why conifers dominate harsh sites
• •Pits in the walls allow water to pass sideways between conduits, which provides detours around blockages
• •Phloem is a separate tissue moving sugars, is made of living cells, and works by pressure rather than tension, running in whichever direction the plant needs
• •Root pressure, generated by active solute uptake, can push water upward modestly and is responsible for droplets appearing at leaf edges in the morning, but it is far too weak to supply a tall tree
• •Ring-barking kills a tree by severing the phloem while leaving the xylem intact, which is why the leaves stay alive for a while and the roots starve

Water under tension is vulnerable to an abrupt phase change. If air enters a conduit through a pit or a wound, the tension pulls it in and the column breaks, forming a bubble that expands to fill the conduit and stops flow through it, which is called embolism or cavitation. Drought and freezing both promote it, freezing because dissolved gases come out of solution as ice forms and seed bubbles on thawing. Plants tolerate a degree of it because the network has many parallel conduits and the pit structures limit the spread of air between them, so the damage is contained rather than propagating through the whole tree. Refilling an embolised conduit against tension is difficult and the mechanisms remain debated, with root pressure able to do it in some species and seasons. This failure sets a hard limit on drought tolerance, and comparative work across species shows that trees generally operate with a narrow safety margin between normal tensions and the point of hydraulic failure, which is one explanation for widespread drought-induced tree mortality: a modest worsening of conditions crosses a threshold rather than degrading performance gradually.

The whole system exists because of an unavoidable conflict. Photosynthesis requires carbon dioxide from the air, which must enter through open stomata, and open stomata inevitably lose water, with hundreds of water molecules lost for each carbon dioxide molecule gained. Plants therefore regulate stomata continuously, opening them to take in carbon and closing them when water stress, darkness or high humidity deficit makes the exchange unfavourable, and the guard cells that do this respond to light, internal carbon dioxide concentration, humidity and hormonal signals from roots detecting dry soil. Several lineages evolved alternative strategies to shift the trade-off, including plants that fix carbon at night when evaporation is low and store it for use during the day, which is what succulents in arid regions do, and a concentrating mechanism in many tropical grasses that allows efficient photosynthesis with stomata less open. Rising atmospheric carbon dioxide alters the arithmetic directly by allowing the same carbon gain with less stomatal opening, which is one of the better-understood effects of the change on vegetation.

Evaporation at the leaf puts the water column under tension, and hydrogen bonding holds the thread together well enough that pulling at the top moves water a hundred metres below, with no pump and no metabolic energy spent on lifting. The conducting cells are dead and reinforced against collapse. Air entering under tension breaks a column and blocks it, which is what kills drought-stressed trees, since most operate near their hydraulic limit.