Make your own transpiring tree

In this paper we present a simple set-up that illustrates the mechanism of sap ascent in plants and demonstrates that it can easily draw water up to heights of a few meters. The set-up consists of a tube with the lower end submerged in water and the upper one connected to a filter supported by a standard filter-holder. The evaporation of water from the filter surface causes a tension large enough to pull the water up the column against gravity. The flow of water leaving the system was visualised by continuously weighing the water container with a balance. The set-up worked for at least 4 days before the water column broke, with typical flows of similar to10 mg min(-1). Knowledge tests with second year university biology students prior to the activity indicated general ignorance not only of the mechanism of sap ascent but also of transpiration itself. After explaining the mechanism of sap ascent with the aid of the set-up all students were able to explain correctly the main aspects of the cohesion-tension theory. The pedagogical value of the set-up, both at the secondary and university levels, is discussed.