Root hemiparasites from the Rhinanthoid clade of Orobanchaceae possess metabolically active glandular trichomes that have been suggested to function as hydathode trichomes actively secreting water. Světlíková et al. conduct macro- and microscopic observations of the leaves of hemiparasitic Rhinanthus alectorolophus and combine them with gas-exchange measurements to estimate the carbon budget of hydathode-trichome activity. They find that night-time rates of respiration and transpiration and the presence of guttation drops are positively correlated, clearly indicating hydathode-trichome activity. Peak activity of hydathodes appears to occur in the juvenile stage before flowering. The results provide the first unequivocal evidence for the physiological role of the hydathode trichomes in active water secretion in the rhinanthoid Orobanchaceae.