Investigation 11: Transpiration

A plant opens its stomata to let CO2 in for photosynthesis, and water vapor escapes through those same open pores. Most of the water a plant's roots absorb is lost this way, not used in building sugars. That loss is actually useful to measure, because it tells you how open the stomata are and how hard the plant is working against conditions like heat, wind, and dry air.

A potometer measures this indirectly. You seal a cut shoot into a tube connected to a water reservoir, with an air bubble trapped in a calibrated capillary tube. As the leaves lose water vapor at the stomata, the whole xylem column gets pulled upward behind it, and that pull drags the bubble along the tube. The distance the bubble travels in a given time, converted using the tube's cross-sectional area, gives you volume of water drawn in, which stands in for water lost at the leaf.

The physical chain behind the bubble's movement is cohesion-tension: water evaporates from cells lining the stomata, lowering pressure in the leaf's xylem; because water molecules hydrogen-bond to each other, that drop in pressure tugs on the molecule below it, and the tug transmits all the way down a continuous water column to the roots. No pump is needed. This links transpiration to water potential and xylem transport elsewhere in the course, and setting up fan, lamp, and still-humid-air trials connects the lab to the broader science practice of designing a controlled experiment, varying one condition at a time and comparing bubble speed across them.