The conduction of sap : I. Water conduction and cavitation in water stressed leaves.

Experiments are described in which the uptake of water by leaves exposed to severe water stress (80-95% R. W. C.) was found to deviate markedly from those of leaves with smaller water deficits. In its most extreme form the deviation appears as a curious increase in rate of water absorption some time after uptake has commenced. It could not be detected during the absorption of water by leaf discs suffering comparable severe water deficits.It is suggested that the differences are caused by cavitation in the xylem conducting channels during wilting. On restoring a water supply the reverse process takes place causing unusual patterns of water uptake.Since restoration takes place at 1° C it appears that metabolism is not involved; the process is physical in character.