Answer: sieve tube elements.
- A sieve tube elements
- B the xylem vessels
- C the tracheids
- D the xylem fibres
Correct answer: A. sieve tube elements
Explanation: Sieve tube elements are living cells that conduct food in the phloem.
Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Concept context
Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.