Mass Flow in Plant Vascular Transport

Steps:

• Identify transport in xylem: water and minerals move via transpiration pull, a form of mass flow from roots to leaves.
• Identify transport in phloem: sugars move via pressure-flow mechanism, involving bulk (mass) flow from sources to sinks.
• Compare options: evaluate which describes a shared feature of both xylem and phloem.
• Select the common process: mass flow applies to both, unlike directionality or energy use.

Why B is correct:

• Mass flow refers to the bulk movement of fluids driven by pressure differences, as defined in the cohesion-tension theory for xylem and pressure-flow hypothesis for phloem.

Why the others are wrong:

• A: Xylem transport is passive, but phloem requires active loading of solutes.
• C: Source-to-sink flow applies only to phloem; xylem moves from roots (source of water) to leaves (sink).
• D: Xylem is unidirectional (upward), but phloem is bidirectional depending on source-sink relationships.

Final answer: B