How does sap flow?
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, which 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 produced, 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, forcing the sap out of the tree. Some of the carbon dioxide also becomes dissolved in the cooled sap. Finally
