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An apparatus designed to measure the rate of water uptake by a cut leafy shoot or whole plant and hence, indirectly, the rate of transpiration. One version consists of a glass tube that bends upwards at one end and supports the plant. The other end of the tube is connected to a capillary tube with an attached scale. The whole apparatus is filled with water from a reservoir joined via a tap to the tube and then made airtight. The plant takes up water and the flow of water is measured by observing the progress of an air bubble along the capillary tube. If the diameter of the capillary tube is known the volume of water taken up by the plant can also be measured. The apparatus is usually used to compare the rate of water uptake when the plant is subjected to certain changes in the external conditions, such as moving air, different light intensities, differences in humidity, etc. It may also be used to compare the rate of water uptake by different plants in the same conditions. An atmometer is a similar apparatus and is used to measure the rate of evaporation from a nonliving wet surface, such as a porous pot. By comparing water loss from a potometer with that from an atmometer under similar conditions, the rate of water evaporation from a leaf, which is controlled by the leaf to some extent, can be compared to the rate of uncontrolled evaporation.
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