IGCSE Biology 0610: Investigating Wind Speed and Humidity Effects on Transpiration

A class investigates transpiration using leafy shoots of the same species and size. Each shoot stands in its own small flask of water, and a thin layer of oil floats on the water surface in every flask. Each flask and its shoot are weighed at the start, then reweighed after one hour, and the mass lost is recorded.

Set-up A: room temperature, still air, low humidity. Mass lost in one hour was 4.8 g. Set-up B: room temperature, air moved by a fan, low humidity. Mass lost in one hour was 6.5 g. Set-up C: room temperature, still air, high humidity (inside a humid enclosed box). Mass lost in one hour was 2.1 g.

(a) Using set-ups A and B, describe the effect of wind speed on the rate of transpiration. [2]

(b) Using set-ups A and C, describe and explain the effect of humidity on the rate of transpiration. [3]

(c) Suggest why the layer of oil on the water surface in each flask was necessary for this investigation to give a valid measure of transpiration. [2]

Part (a): Effect of wind speed

Set-ups A and B are kept at the same temperature and humidity, and differ only in whether the air is still or moving. Set-up B, with air moved by a fan, lost 6.5 g, more than the 4.8 g lost by set-up A with still air. Since wind speed is the only factor that differs between them, this shows that a higher wind speed increases the rate of transpiration.

Part (b): Effect of humidity

Set-ups A and C are kept at the same temperature with still air, and differ only in humidity. Set-up C, in the humid enclosed box, lost only 2.1 g, less than the 4.8 g lost by set-up A in drier air. This shows that higher humidity decreases the rate of transpiration.

The reason is that transpiration depends on water vapour diffusing out of the leaf’s air spaces through the stomata, and diffusion happens faster when there is a bigger difference in concentration between two regions. In humid surrounding air, the air already holds a lot of water vapour, so the difference in water vapour concentration between the air spaces inside the leaf and the air just outside the stomata is small. This smaller concentration difference means water vapour diffuses out more slowly, so the shoot loses less water overall.

Part (c): Why the oil layer was needed

Each flask’s water surface is directly exposed to the air, and water could evaporate from this open surface just as it would from any container of water, regardless of anything happening in the shoot. The layer of oil floating on top forms a barrier that stops this direct evaporation from the water surface. This means that any mass lost from a flask can be confidently attributed to water lost by the shoot through transpiration, rather than partly to evaporation from the flask itself, which is essential for the comparison between set-ups to be a fair and valid one.

Final answers

(a) Wind speed increases the rate of transpiration (B lost more than A).
(b) Higher humidity decreases the rate of transpiration, because a smaller water vapour concentration difference between leaf and air slows diffusion out through the stomata (C lost less than A).
(c) The oil layer prevents evaporation from the flask’s water surface, so all recorded mass loss is due to the shoot’s transpiration, keeping the comparison valid.