SUMMARYCavitation has been studied by acoustic detection in preserved and fresh vascular bundles and in whole leaves of Plantago and other plants. 'Clicks', indicating cavitation vibrations, were produced as vascular bundles lost water in a manner similar to that of Ricinus leaves (Milburn, 1973a), but the process was more rapid and freely suspended bundles jerked as they cavitated, like spore elator systems. Clicks were counted in a semi-quantitative study of both cavitation and the reverse process, the restoration of water-filled xylem conduits. Restoration was greatly enhanced by compression or vacuum infiltration treatments in comparison with simple soaking: apparently gas entrainment is the major obstacle preventing refilling.Cavitation within wilting leaves was much less extensive than when isolated bundles are dried, indicating that cavitation is generally incomplete under natural conditions. Xylem sap tensions at which cavitation is incipient were estimated approximately by balancing cavitation against the external humidity. Sap tensions at incipient cavitation could be measured more accurately using a pressure bomb, however. These tensions were surprisingly low, covering a range of tensions (5-15 bar) expected to occur under field conditions. Cavitation was detected in Plantago under natural field conditions and in other plants also, especially Tussilago. Further implications are discussed.