Phloem anatomy — Which cell types make up the functional conducting tissue of phloem in angiosperms?

Introduction / Context:
Phloem transports photosynthates and signaling molecules throughout the plant. Understanding its cellular composition is vital to grasp loading/unloading, pressure flow, and response to injury (callose deposition at sieve plates).

Given Data / Assumptions:

• Angiosperm phloem comprises sieve-tube elements associated closely with companion cells.
• Sieve plates connect sieve-tube elements into long conducting files.
• Companion cells support metabolic functions lost by sieve-tube differentiation.

Concept / Approach:
Sieve-tube elements are living, enucleate conduits specialized for mass flow of sap. Companion cells (derived from the same mother cell) maintain the metabolic machinery, regulate loading/unloading, and communicate via plasmodesmata. Xylem cells (tracheids, vessel elements) are dead at maturity and are not part of phloem.

Step-by-Step Solution:

Verification / Alternative check:
Microscopy reveals sieve plates and dense companion cell cytoplasm; functional studies show companion cell roles in sucrose loading and signaling.

Why Other Options Are Wrong:

• Vessel elements/tracheids: xylem components.
• Tracheids with sieve-tube elements: mixes xylem with phloem.
• Parenchyma/fibers alone: lack sieve function.

Common Pitfalls:
Confusing sieve cells/albuminous cells (gymnosperms) with angiosperm sieve-tube/companion cell pairs.

Final Answer:
Sieve-tube elements and companion cells