Cambridge O Level Biology · Syllabus 5090 · Transport in Flowering Plants
Xylem Vessel
What is Xylem Vessel?
A long, hollow tube in the vascular tissue of a plant, formed from a file of cells joined end to end whose cross walls have broken down and whose cell contents have been lost, so that the mature vessel is dead and its lumen is continuous. Its walls are thick and strengthened with lignin, which both supports the plant and keeps the vessel open. Xylem vessels carry water and dissolved mineral ions in one direction, from the roots to the leaves, drawn upwards by the transpiration pull.
This definition is part of the Transport in Flowering Plants chapter in Cambridge O Level Biology.
Common mistakes with Xylem Vessel
- 5. “Xylem vessels pump the water up.” Why wrongA mature xylem vessel is dead and empty. It has no membrane, no cytoplasm and no mitochondria, so it cannot do anything that requires energy. WriteWater is drawn up the xylem by the transpiration pull, a tension created when water is lost from the leaves.
- 13. “It does not matter whether the shoot is cut in air or under water.” Why wrongCutting in air lets air into the open xylem vessels. The water column is broken, the transpiration pull cannot be transmitted past the air lock, and uptake falls or stops. WriteThe shoot is cut under water so that air does not enter the xylem and break the continuous water column.
Questions students ask about Xylem Vessel
Why must a potometer shoot be cut under water?
To stop air entering the xylem vessels. An air bubble inside the xylem would break the continuous column of water, so the transpiration pull could not be transmitted and water uptake would slow or stop, giving a falsely low reading.