Xylem transport — The cohesion–tension theory primarily explains water movement through which conducting tissue in vascular plants?

Introduction / Context:
The cohesion–tension theory is the dominant model for ascent of sap, describing how water moves in a continuous column from roots to leaves driven by transpiration and supported by cohesive and adhesive forces within xylem conduits.

Given Data / Assumptions:

• Water evaporates from mesophyll cell walls, creating tension (negative pressure) in leaf xylem.
• Hydrogen bonding leads to cohesion between water molecules and adhesion to xylem walls.
• Xylem tracheids and vessel elements are dead, lignified tubes optimized for tension transmission.

Concept / Approach:
Transpiration at the leaf surface pulls on the water column. Because the xylem network is continuous from roots to leaves and water molecules cohere, this tension is transmitted downward, drawing water upward from the roots and ultimately from the soil. The theory specifically pertains to the xylem pathway, not to phloem or guard cell mechanics.

Step-by-Step Solution:

Verification / Alternative check:
Cavitation/vulnerability curves and pressure chamber measurements support tension-based transport; cutting stems breaks the continuous column and can cause embolisms, consistent with the model.

Why Other Options Are Wrong:

• Phloem: transports sugars via pressure flow, not cohesion–tension.
• Guard/companion cells: regulate stomata or phloem loading, not bulk water ascent.
• “From leaf to root only”: water flow direction is root to leaf in xylem under transpiring conditions.

Common Pitfalls:
Assuming root pressure drives most ascent; it is minor compared with transpiration-driven tension in most conditions.

Final Answer:
Xylem