DEMONSTRATION OF SOME OF THE MECHANISMS INVOLVED IN THE ASCENT OF SAP IN PLANTS

The usual demonstration of osmosis with sugar solution separated from water by a differentially permeable membrane is unsatisfactory as a model representing the plant mechanisms involved in sap ascent. Beginning students are often confused because in the demonstration they see a solution rising in a tube due to a force exerted not from above, as it is in the plant, but from below. A simple rearrangement of the materials ordinarily used provides a very convincing demonstration of the process of osmosis, and at the same time shows more exactly how osmosis in the leaf cell brings about the rise of water in the stem below. The addition of a capillary evaporating, surface to the apparatus then makes the picture of the mechanisms involved and their coordination quite complete. Figure 1 (A) shows how the apparatus is arranged. A glass tube (t), one or more meters long, communicating above with a short glass cvlinder