Identifying phloem sieve tube features in electron micrographs

Steps:

• Recognize sieve plate as a hallmark of phloem sieve tubes in plants.
• Note that mature sieve tubes have thin peripheral cytoplasm and lack nucleus or large vacuole.
• Identify mitochondria as abundant in associated companion cells for energy needs.
• Match labels to sieve tube characteristics: sieve plate, thin cytoplasm, mitochondria.

Why A is correct:

• Phloem sieve tubes contain sieve plates for transport, thin cytoplasm lining the cell wall, and mitochondria in companion cells, per plant cell ultrastructure.

Why the others are wrong:

• B: Mature sieve tubes lack prominent vacuoles, which dominate in other cell types.
• C: Mature sieve tubes are enucleate, losing their nucleus during differentiation.
• D: Sieve tubes do not contain chloroplasts, which are found in photosynthetic cells.

Final answer: A