DEMONSTRATING THE LIFTING POWER OF TRANSPIRATION

Abstract

Transpiration and the cohesion of water are two important factors in the rapid rise of water in the conducting systems of stems. The problem of demonstrating these two factors has led to numerous experiments. In the present investigation several modifications of procedure have been devised and have been proved satisfactory. Living leafy twigs have been used to pull mercury columns to heights exceeding barometric pressure. Previous investigators have used two general methods to demonstrate the plausibility of the cohesion theory of the rise of water in plants. In the first method a purely artificial system was used, the apparatus consisting of a porous evaporating surface attached to the top of a long glass tube. First the porous material and tube were thoroughly wetted and then the whole apparatus completely filled with water. The lower end of the glass tube was then dipped into a vessel containing mercury. On allowing the water to evaporate from the porous surface the volume of water decreases, and the mercury is pulled up the tube by the retreating water column. If the mercury column is lifted to a height exceeding barometric pressure then a definite pull is shown to exist in the water column. This experiment has been used to explain the rise of water in plants. The porous material is thought of as the leaf and the glass tube an element in the water-conducting system of the plant. The water is thus pulled up the trunk of the plant by the cohesive properties of the water coluimn. Askenasy (I896), Ursprung (1913), Lubin (1926), Thut (1928), Otis (1930), and others have attempted to perfect such a set-up. The writer has described, in his paper just cited, an experiment in which a column of mercury was lifted 226.6 centimeters. This is equivalent to lifting a column of water approximately I00 feet. In the second method actual living material was used as part of the apparatus. Leafy twigs were employed as evaporating surfaces. Only one successful set-up of this type, that of Boehm's (I893), has been reported in literature. His work, though often cited, has been neglected and probably doubted. Boehm successfully attached living Thuja twigs to capillary manometer tubes and by transpiration and cohesion of water lifted mercury columns to heights of 86.4 and 90.6 centimeters. By this method students of botany need not use an analogy, but can see by actual demonstration that a transpiring leafy twig does exert a pull on a water column.