AbstractOscillations in transpiration and water uptake of individual, young oat plants have been studied. The free‐running period of these oscillations was about 30 minutes. Conditions were reached under which the oscillations were sustained for about two days.Short perturbations were given to the transpiration oscillations, the perturbations consisting of a short time increase or decrease in the illumination. The phase shifts of the oscillations as well as the amplitude effects caused by these perturbations were investigated.Simultaneous recordings of transpiration and water uptake of a single plant showed that these functions were oscillating in phase. Both oscillations disappeared if the leaf was excise