Transpiration is the loss of water vapour from a plant’s leaves, mainly through the stomata. A potometer measures how quickly a cut shoot takes up water. This gives an estimate of transpiration rate, because water uptake is not necessarily equal to water loss.

How transpiration happens

Water evaporates from the moist surfaces of mesophyll cells into air spaces inside the leaf. Water vapour then diffuses out through the stomata, which are pores in the leaf surface. This loss of water helps pull more water up the xylem from the roots. The pull is transmitted through the continuous column of water in the xylem because water molecules cohere to one another.

How a potometer is used

A typical bubble potometer holds a cut leafy shoot in a water-filled, airtight apparatus connected to a narrow tube. An air bubble is introduced into the tube. As the shoot takes up water, the bubble moves towards it. Measure the distance the bubble travels in a set time, then calculate:

taken\text{rate of bubble movement}=\frac{\text{distance moved}}{\text{time taken}}

For example, if the bubble travels $mm30\,\mathrm{mm}$ in $min5\,\mathrm{min}$ , its rate is $min−16\,\mathrm{mm\,min^{-1}}$ . If you know the tube’s cross-sectional area, you can instead calculate the volume of water taken up per unit time.

Cut the shoot under water to reduce the chance of air entering the xylem, and check that the apparatus is airtight. To investigate a factor such as light intensity, change that factor while keeping other conditions, such as temperature and air movement, as constant as possible.

Exam mistake to avoid: do not say that a potometer directly measures water vapour leaving the leaves. It measures water uptake; some of that water may be used by the plant or help maintain cell turgor.