Water and dissolved minerals are taken up through cells called the xylem, and food manufactured in the leaves is distributed to the rest of the plant through cells called the phloem.
The rise of sap is essentially concerned with the xylem.
It is a system of very fine tubes that continuously connect the extremities of any plant with the tips of the roots, where the water and minerals are absorbed. The tubes consist of individual cells arranged end to end, with one or more openings at the end.
These capillary tubes are so fine that the molecules of water within them move through the entire tube as a single unit.
The leaves have pores for the exchange of gases necessary for photosynthesis. When the pores are open, there is a loss of water, known as transpiration. As water is lost, a pull is exerted on the column of water.
Two forces are at work to sustain this tremendous pull and keep the stream of water intact.
The first is the adherence of water molecules to the cell wall, which can occur only in a very fine tube. The second is the strong coherence of water molecules to each other.
In experiments, these forces of adhesion and cohesion are more than strong enough to carry water as high 05 the tallest trees, the California redwoods, 350 feet or more.
The sap we call maple syrup is a special case involving stern pressure.
In daytime in late fall through spring, when the leaves are not out, cells in the stem start metabolizing.
The process, which is not fully understood, produces carbon dioxide, which collects in the spaces between the cells. The pressure forces the sap out when a hole is made.
At night, the carbon dioxide is dissolved into the sap, creating a small vacuum, which pulls more water in from the roots.
Once leaves start coming out in the spring, the normal circulation takes over, the pressure in the stem is not as great, and the maple syrup stops running.
