Ascent of sap in trees.

Experimental results concerning the ascent of sap in the xylem are usually interpreted in terms of gradients of hydrostatic pressure in the xylem conduits. In this report an alternative model is proposed that is equally consistent with the experimental results: under static conditions the water column is supported by a gradient in the chemical activity of the water, and the hydrostatic pressure is constant throughout. Observations that support the new model are cited, and experiments are suggested that would permit a choice between the two models.