Which of the following statements about plant vascular tissue is correct?

Question

BlackTom AI · Accepted Answer

Xylem conducts water and dissolved minerals upward from roots to shoots in a single, predominantly unidirectional stream.

1. Relevant knowledge points and concepts
- Xylem: vascular tissue made of dead tracheids and vessel elements that transport water and minerals from roots to aerial parts. Movement is driven mainly by transpiration-cohesion-tension and sometimes root pressure. Because xylem elements are non-living and structurally adapted for upward flow, this transport is effectively one-way (root → shoot).
- Phloem: composed of living sieve-tube elements and companion cells that transport organic solutes (sugars, amino acids, hormones). Phloem translocation follows the pressure-flow (mass flow) mechanism and moves from sources (where sugars are produced or released) to sinks (where they are used or stored). Direction can be up or down depending on which tissues are sources or sinks.

2. Step-by-step reasoning
- Water and mineral uptake: Roots absorb water and dissolved minerals from soil into xylem vessels.
- Driving force: Transpiration from leaf stomata creates negative pressure at the leaf end of the xylem, pulling a continuous column of water upward via cohesion of water molecules and adhesion to xylem walls.
- Unidirectionality: Because the xylem pathway and driving force (transpiration) consistently move water from root to shoot under normal conditions, xylem transport is effectively single-direction.
- Phloem contrast: Sugars loaded into phloem at source tissues create a high osmotic pressure; unloading at sinks lowers pressure; flow direction depends on relative locations of sources and sinks, so phloem transport is bidirectional.

3. Intermediate steps / mechanisms (no numeric calculations required)
- Transpiration → negative pressure in leaf xylem → cohesion-tension pulls water column → root uptake replenishes water.
- Phloem loading at source → water influx raises pressure → mass flow toward sink → unloading reduces pressure.

4. Why the correct answer is correct
The statement "The xylem transports water and minerals from the roots to the shoots in a single direction" accurately describes the structural features and primary driving mechanism of xylem transport.

5. Why other options are incorrect
- "Both xylem and phloem transport water, minerals and sugars in both directions." Incorrect: xylem does not transport sugars and is essentially unidirectional; phloem can transport sugars but not always in both directions simultaneously along the same strand.
- "The phloem transports sugars from the leaves to the roots in a single direction." Incorrect: phloem transport is source-to-sink and can be upward or downward depending on plant developmental stage and metabolic demands.
- "Phloem cells transport water in a single directions to leaves." Incorrect: phloem primarily transports organic solutes; water movement in phloem accompanies solute flow but is not a unidirectional transport to leaves.

6. Conclusion
The correct answer is: "The xylem transports water and minerals from the roots to the shoots in a single direction." This matches the provided answer.

Which of the following statements about plant vascular tissue is correct?单项选择题

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