Ascent of Sap in Plants

Abstract

1. These studies are concerned with the ultimate cause of the transpiration stream in the xylem tissue of vascular plants. 2. The current theories of sap rise involve active secretion of water by living cells of the leaves, excepting at times when the evaporation rate exceeds the secretion rate of these cells. 3. Repetitions of Dixon's experiments on sap rise with submerged shoots, and shoots transpiring into a saturated atmosphere without first satisfying the saturation deficit of the shoots, gave results in agreement with his findings. But in shoots in which the saturation deficit was first relieved there was no evidence of secretory activity of the leaf cells, and no significant rise of sap. The phenomena observed by Dixon are therefore believed to have been caused by saturation deficits in the twigs and shoots existing at the time the experiments were performed. 4. Although no experimental evidence is presented, it is believed that downward translocation of dye in the xylem, when leaves on wilty plant...