Transpiration

What is transpiration? — Core

Water travels up the xylem and reaches the mesophyll cells inside a leaf, but it doesn’t all stay there. Some of it evaporates from the surfaces of the mesophyll cells into the air spaces that run through the leaf’s interior — at this point, liquid water has become water vapour. That vapour then diffuses through the internal air spaces and out of the leaf through the stomata, down its own concentration gradient into the drier air outside.

The required sequence is: water at the mesophyll cell surfaces → evaporation → water vapour in the leaf’s air spaces → diffusion through the stomata → the atmosphere outside. Cambridge’s current wording for the definition itself is specific — transpiration is the loss of water vapour from leaves— and it’s worth sticking to that exact phrasing rather than looser variants like “water loss from leaves and shoots,” and worth remembering that ordinary transpiration doesn’t involve liquid water simply flowing out of an open stoma.

Why leaves can lose water so readily — Supplement

A leaf’s internal structure is built for gas exchange, and that same structure happens to make rapid water loss possible too. The interconnecting air spaces between mesophyll cells expose a very large internal surface of moist cell wall to the air inside the leaf, and a larger exposed wet surface simply means more area from which water can evaporate. What happens next depends on the stomata themselves: the number of stomata affects how many exits are available for that water vapour, and the size to which they open affects how easily vapour can pass through each one. A leaf with more or larger open stomata, other things being equal, loses water faster than one with fewer or smaller openings — the same architecture that makes efficient gas exchange possible is also the architecture that makes transpiration hard to avoid.

Transpiration pull — Supplement

Water lost from the leaf during transpiration doesn’t just vanish from the plant’s point of view — it’s replaced by water drawn up from the xylem below, and that replacement is the basis of transpiration pull. Water molecules attract one another, and because of that attraction the water inside a xylem vessel behaves as a single continuous column rather than a series of separate droplets. When water is drawn out of the top of that column into the leaf, the attraction between molecules pulls the water immediately below it upward too, and that effect is transmitted all the way down the column.

The full sequence: water evaporates from the mesophyll cells → water vapour leaves through the stomata → replacement water is drawn from the xylem → transpiration pull acts on the water column below → water moves upward through the xylem. This continuous upward movement is usually called the transpiration stream. It’s worth being precise about what is nothappening here: there is no pump inside the plant pushing water up from the roots, and evaporation shouldn’t be described as though it were actively pumping water upward — the actual mechanism is a pull transmitted through a cohesive column of water molecules. Detailed root pressure and the full physics of cohesion–tension theory go beyond what this syllabus point requires.

Wilting — Supplement

Plant cells rely on internal water pressure, turgor, to stay firm, and wilting is what happens when that pressure can’t be maintained. If water is lost through transpiration faster than it can be replaced by uptake at the roots, cells begin to lose water, and their turgor pressure decreases as a result. As cells become flaccid rather than turgid, young stems and leaves — which depend heavily on turgid cells for support — lose their rigidity and begin to droop. That drooping is wilting.

The sequence is: water loss exceeds water uptake → cells lose water → turgor pressure decreases → cells become flaccid → leaves and young stems lose support and droop. It’s worth resisting the shortcut of explaining wilting as simply “the plant got hot” — heat can certainly increase the rate of transpiration, but the actual biological cause of wilting is the imbalance between water loss and water uptake leading to loss of turgor, and that’s the causal chain an examiner wants to see spelled out. Wilting caused this way can usually be reversed if the plant receives enough water before its tissues are seriously damaged.

Cross-links

The water reaching the mesophyll cells at the start of this page follows the pathway described on the water uptake page. What actually changes the rate of the process described here — temperature, wind speed and humidity — along with how Cambridge expects that rate to be investigated with a potometer, is covered on factors affecting transpiration.