STREAMLINE FLOW AND THE MOVEMENT OF SOLUTES IN THE TRANSPIRATION STREAM

The classic experiment of ASKENASY (1, 2), illustrating the pull exercised by an evaporating water surface, consists, as is well known, of a thistle funnel closed at the wide end by a plug of plaster of paris, made airtight, if necessary, by ringing with gold-size. The funnel is filled with well boiled distilled water, and inverted into a vessel of mercury. As the water evaporates from the upper surface of the plaster plug the mercury rises pari passt at the lower end of the tube, and might be expected to reach eventually barometric height, less the vapor pressure at the prevailing temperature. Actually the surface tension and the molecular cohesion of the water draw the mercury to much greater heights. ASKENASy observed a rise of 89 cm. at a barometric pressure of 73.5 cm. URSPRUNG, in a series of papers (7, 8, 9, 10, 11), made more extensive observations and obtained a rise of 135 cm. above barometric height, in glass tubes, and of double barometric height in liane stems used as tubes. In all cases, bubbles are formed eventually and the cohesion breaks down. The rate at which the mercury rises depends, of course, primarily on the rate of evaporation and secondarily on the ratio between the area of the evaporating disc and the cross section of the tube. The rate, however, is not exactly equivalent to the rate of removal of the water. NORDHAUSEN (3) has shown that an evaporating surface can raise water from a vacuum, i.e., develops beneath it a negative pressure of at least one atmosphere. Our own experimental comparisons of the weight/volume ratio during the rise of the mercury column showed a diminution in the density of the water of as much as 10 per cent., so that as the experiment proceeds an increasing negative pressure develops in the water, long before barometric height is reached, which tends to withdraw the water menisci from the pores of the evaporating surface and thus check the rate of flow. The existence of this negative pressure can readily be shown by using a thistle funnel with plaster plug, the stem of which dips into mercury in a closed filter flask. The flask is partly evacuated and time is then allowed for a negative pressure to establish itself and for the mercury to rise a short distance in the stem of the funnel. On admitting air to the flask the mercury rises with a sudden bound upward, often to twice its previouLs height, in replacement of the contracted water column. 773