For many years most plant physiologists have been pushing a theory of sap flow called the cohesion hypothesis. This theory has never definitely been shown to be the complete answer, but over a long period of time, since nothing better was offered, has gradually come to be completely accepted (chapter 4 of the book Plant Physiology by Salisbury and Ross 1985).

Recently there has been a revival of research that limits the usefulness of the usual hypothesis (e.g. Zimmermann et. al. Annals of Botany volume 76 pages 545-551, 1995). No theory, published in the current plant physiology literature, seems to completely solve the problem.

At Wagner Research Laboratory we used a completely different experimental approach to find out how sap flows. We cut tiny holes in the sap conducting tissue (xylem) of trees. We placed small metal encased accelerometers in these holes. The accelerometers, isolated from the tree tissue, indicate that there are forces present, when the sap is flowing, that at least partially cancel gravity to facilitate vertical sap flow. Similar forces were found to facilitate horizontal sap flow. This discovery limits the application of the usual hypothesis. The Wagner accelerometer results have been published in Physiological Chemistry and Physics in 1992 and 1995 with another article published in 1996 (see selected references). The Wagner book Waves in Dark Matter brings most of the material on this subject together.

Gravity like forces provide a good explanation for sap flow up tall trees. These forces appear to complete the cancellation of gravity and other resistance to the flow of sap and can be changed, as necessary, to accommodate the needs of the plant. The forces found may be related to moving standing waves. (see the Wagner article A Basis for A Unified Theory for Plant Growth and Development and read the book Waves in Dark Matter).