Amber Gravott, age 13, of Hattesburg, Miss., for her question:
HOW DOES SAP MOVE IN A TREE?
In biology, sap is the liquid in the stems and roots of all plants, including trees. There are two kinds of sap: one is water with dissolved minerals that travels from the roots to the leaves and the other is water carrying dissolved plant food that moves from the leaves to parts of the plant where it is to be used or stored.
Water with dissolved minerals moves through a layer in the stem and trunk called the xylem. Water with dissolved plant food moves in a layer called the phloem.
The xylem sap movement starts near the growing tip of the root, where water and minerals enter the plant. From the region of the root hairs, the water with the minerals dissolved in it moves toward the inside of the root by diffusion.
When the sap reaches the xylem layer in the root, it moves up the root into the xylem of the stem. Finally it passes to the leaves.
The force that causes this sap to rise is, to a great extent, a pull from above. The pull results from the transpiration or evaporation of water from the leaves.
In young trees, the water moves through the xylem in all parts of the wood. But in old trees, the only part of the xylem that transports sap is the part near the bark. This part of the wood is called sapwood.
The part of a tree through which the sap does not move is called the heartwood.
It is usually possible to tell the two kinds of wood from each other by their color. Sapwood is usually light in color, while heartwood is usually much darker.
The leaves are the organs of the plant that manufacture carbohydrates from carbon dioxide and water. The plant then uses the carbohydrates as food. Carbohydrates dissolve in water and form the other kind of sap.
In the stems and trunk of the plant or tree, this stream of sap moves downward.
The force that causes the downward movement of phloem sap is probably. the high osmotic pressure. This pressure results from the movement of water into the sugar rich cells of the leaves.
The pressure passes to the conducting cells of the phloem, and a column of sap is forced downward.
At certain times of the year, in some trees, the stream moving upward may also contain carbohydrates. An example of this is the sugar maple, whose rising sap contains sugar.
The sap of many plants is valuable to man. Most sugar sold in stores is made from the sap of sugar cane. The sugar maple also is a source of sugar and syrup.
Plants contain other liquids that are not true sap, in the botanical sense. The milky juice of milkweed is an example. Many milky juices have great value. The milky juice of rubber trees becomes the raw material for natural rubber. Gums and resins are also valuable plant juices.