Companion cells facilitate phloem transport by supporting sieve tube elements

Steps:

• Recall phloem structure: sieve tube elements lack nuclei and rely on companion cells for metabolic support.
• Identify companion cell role: they connect via plasmodesmata to load sugars into sieve tubes.
• Evaluate choices: match functions to known biology of phloem unloading and transport.
• Confirm A as primary function based on symplastic loading mechanism.

Why A is correct:

• Companion cells provide cytoplasmic connections through plasmodesmata, enabling active loading of assimilates into sieve tube elements for translocation, as defined in phloem physiology.

Why the others are wrong:

• B: Structural support is provided by sclerenchyma fibers, not companion cells.
• C: Companion cells have their own nuclei; they do not supply nuclei for division in sieve tubes.
• D: Assimilates originate from photosynthetic sources like leaves, not companion cells.

Final answer: A