Structure of transport tissues — practice questions

1. Continuous lumen: Xylem vessels are hollow, continuous tubes with no end walls, allowing for an uninterrupted column of water and reducing resistance to flow.
2. Lignified walls: The walls are thickened with lignin, providing mechanical strength to withstand the negative pressure (tension) generated during transpiration, preventing the vessels from collapsing.
3. Waterproofing: Lignin also makes the walls impermeable to water, ensuring that water moves through the lumen of the vessel rather than diffusing out.
4. Narrow diameter: The relatively narrow lumen of xylem vessels contributes to the capillary action and maintains a continuous water column, aiding in the upward movement of water.

1. Actual size $= \frac{\text{image size}}{\text{magnification}} = \frac{8\,\text{mm}}{400}$.
2. $= 0.02\,\text{mm} = 20\,\mu\text{m}$.

The narrow lumen reduces resistance while maintaining capillary action in the transpiration stream.