During warm periods when temperatures rise above freezing, pressure develops in the tree. This pressure causes the sap to flow out of the tree through a wound/tap hole. During cooler , suction develops, drawing water into the tree. This replenishes the sap in the tree, allowing it to flow again during the next warm period.
Sap flows through a portion of the outer tree trunk called sapwood. Sapwood consists of actively growing cells that conduct water and nutrients(sap) from the roots to the branches of the tree. During the day, activity in the cells of sapwood produces carbon dioxide. This carbon dioxide is released to the intercellular spaces in the sapwood. In addition, carbon dioxide that was dissolved in the cool sap is released into the spaces between the cells. Both of these sources of carbon dioxide cause pressure to build up in the cells. A third source of pressure is called osmotic pressure, which is caused by the presence of sugar and other substances dissolved in the sap. When the tree is wounded, as when it is tapped by a maple producer, the pressure forces the sap out of the tree. At night or during other times when temperatures go below freezing, the carbon dioxide cools and therefore contracts. Some of the carbon dioxide also becomes dissolved in the cooled sap. Finally, some of the sap freezes. All three of these factors create suction in the tree. This causes water from the soil to be drawn up into the roots and travel up through the sapwood. When temperatures rise above freezing the next day, sap flow begins again.
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