Features of sieve tube elements in phloem

Steps:

• Sieve tube elements form phloem tissue for sugar transport and are living cells with thin, non-lignified walls.
• They contain cytoplasm for mass flow but lack nuclei and most organelles.
• Plasmodesmata connect them to companion cells and through sieve plates for symplastic continuity.
• Lignified walls, mitochondria, and chloroplasts are absent, as these cells rely on companion cells for metabolism.

Why C is correct:

• Sieve tube elements are defined as enucleate, living phloem cells with cytoplasm and plasmodesmata for efficient transport without rigid support or photosynthesis.

Why the others are wrong:

• A includes lignified walls, characteristic of xylem vessels for water conduction, not phloem.
• B includes lignified walls and mitochondria/chloroplasts, which sieve tubes lack due to organelle degeneration.
• D includes chloroplasts, absent in sieve tubes as they do not perform photosynthesis.

Final answer: C