Tree plumbing

To survive and flourish, trees need to transport water upwards from root to crown. They do so using transpiration, capillary action and osmosis.

Transpiration is the process by which water moves up the xylem’s thin hollow tubes and evaporates through their leaves and flowers. In mature trees this can total thousands of litres daily.

The strong bond of water molecules enables the water columns that make this suction possible to exist. Cohesion is the force that bonds molecules of a single substance. In trees it enables a continuous stream of water to form in the vessels.

Conversely, adhesion is the force that bonds separate substances. In trees this applies to water molecules as they bond to cellulose in the walls of xylem cells.

Capillary action is the force that makes the water surface higher around the edge (like in your cuppa). The narrower the vessel, the stronger the action. However, its impact is relatively minor.

Osmosis in trees is the equalisation of sugar levels between adjacent cells. It occurs cell-to-cell, drawing water from the roots and up the crown.

The tallest trees (e.g. redwoods) need to move water 100m or more vertically upwards – about 30 storeys. The necessary suction is created by evapotranspiration – the combined effect of evaporation and transpiration.

In deciduous hardwood trees the hollow transportation columns in the xylem, called vessel elements, are made up of groups of cells.

Coniferous softwoods transport water through tracheids which are made up of single cells and are over 25 times narrower than deciduous vessel elements.

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Image: GeeksforGeeks