Tim Bell, age 12, of Longview, dash., for his question:
WHAT MAKES SAP IN A PLANT MOVE?
Sap is the liquid in the stems and roots of plants. There are two kinds of sap. One kind is water with dissolved minerals, that travels from the roots to the leaves. The other kind of sap is water carrying dissolved plant foods. It moves from the leaves to parts of the plant where it is to be used or stored.
The sap that is water with dissolved minerals moves through a layer in the stem and trunk called the xylem. The sap that is water carrying dissolved plant foods 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 water reaches the xylem layer of the root, it moves up the room into the xylem of the stem. Finally it passes to the leaves. The force that causes the sap to rise is, to a great extent, a pull from above. The pull results from the transpiration or evaporation of the 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.
The leaves are the organs of the plant which 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, this stream of sap moves downward. The force which 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 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 is also a source of sugar and syrup.
Plants contain other liquids that are not true saps, 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 also are valuable plant juices.
Ninety five percent of the sap that rises in a tree is evaporated into the air by the leaves of the tree. Only two percent is used to dissolve plant food and returns down the tree.
The force that raises the sap is strong enough to lift it to the top of a tree over 300 feet high.
An average sized apple tree can lift and distribute four gallons of sap an hour.