How does water reach a plant’s leaves?

A leaf high in a tree can receive water that was in the soil below it. The tree has no heart pushing that water up its trunk. Much of the driving effect begins at the other end of the journey, where water escapes from leaves into the air.

Roots take in water from moist soil. Inside the root, water reaches xylem, the tissue that carries it through the plant. Many of its conducting cells form long, narrow, interconnected passages. Their strengthened walls help them resist collapse as water is pulled through them. The route continues through stems and branches into the veins of leaves.

Inside a leaf, water evaporates from moist cell walls into air spaces. Water vapor then escapes through small pores called stomata. This loss is transpiration. As evaporation draws water from those moist surfaces, it creates tension in the connected liquid supply. Tension is a pulling stress: the water column is being drawn toward the leaf, rather than being pushed upward by a pumping organ.

Water can transmit that pull because neighboring water molecules attract one another, a property called cohesion. Water also adheres to the surfaces of the conducting passages. Together with the narrow pathways and evaporation at the leaf, these properties allow a continuous liquid route to connect soil and canopy. Capillary rise alone would reach a limited height in an ordinary tube; ongoing transpiration is central to supplying a tall tree.

This movement depends on the surroundings. Dry air and moving air can increase water loss, while humid air can slow it. Roots must have water available to replace what leaves. Guard cells adjust stomatal openings, helping the plant balance water loss with the carbon dioxide entry needed for photosynthesis. Closing the pores conserves water but also restricts that gas exchange.

Much of the water taken up eventually returns to the atmosphere as invisible vapor. Along the way, it carries dissolved minerals, supplies growing tissues and helps cool leaves as it evaporates. A tree's water route therefore connects the ground beneath our feet to the air above our heads. A leaf is both a place where the plant uses water and a place where water loss helps keep the whole stream moving.