The Water Content of Maple Stems. II. Apparent Changes in the Water Content of Maple Stems and the Physiology of Maple Sap Flow.

Abstract

For manv vears the mechanism producing a vessel sap pressure in Acer and some other genera has attracted the attention of a few investigators. Early studies demonstrated that a rise in the temperature of dormant stems from just below or just above freezing, often induced vessel sap pressures and a flow from a wound in the stem. A decrease in stem temperature reduced the pressures and the flows. The pressure-inducing mechanism was recognized as a characteristic of the stem by Jones et al (1) and by Wiegand (7). Jones et al (1) measured vessel sap pressures and found that during the cooling part of the diurnal cycle these pressures were below one atmosphere. They also studied the direction of the pressure and found that during a flow the larger pressure was in a downward direction. The pressureinducing mechanism was also studied by Stevens and Eggert (6). They observed the absorption of water by excised stems and recognized that the role of the roots was to supply sap to the stem during the cooling period. Marvin and Greene (5) measured the amounts of absorption and flow by excised stems under conditions of controlled temperature changes. The quantitative relationship between temperatures and sap flow in intact plants was studied by Marvin and Erickson (4). They presented evidence that a quantitative relationship of temperature to the flow could be shown for the cooling part of the diurnal