Answer: sieve tubes.
- A sieve tubes
- B xylem vessels
- C tracheids
- D xylem fibres
Correct answer: A. sieve tubes
Explanation: Each sieve tube element is supported by an adjacent companion cell.
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.