The Cohesion-Tension Theory of Sap Ascent

Abstract

The cohesion-tension theory is also sometimes called the cohesion theory. It is a rather old theory dating back to the late nineteenth century, which attempts to explain the principal forces that govern the ‘ascent of sap’ from the soil to the leaves in plants. It was a very controversial theory when first proposed (Böhm 1893; Dixon 1914, pp. 142–154) and it has been the focus of controversy about every 20 years since it was postulated. At the time of writing, there has been more controversy over the cohesion theory than at any other time, excluding the initial controversy when it was originally proposed. The theory, at the very least, is incomplete in the sense that it describes a ‘universal’ motive force for water movement in plants. This should not surprise us because nineteenth century science had an incomplete understanding of what makes water move in plants because the science of thermodynamics was not mature enough to provide plant physiologists with the necessary insights. While the cohesion-tension theory is not perfect and has a few unsolved problems, I personally feel much of the current debate has proved to have little merit, as will become evident in this and subsequent chapters.