Question

While xylem is always open and used for transport of water, phloem is open only when the food needs to be transported. Which of the following reasons supports this statement?

• A. Sieve tubes have companion cells that control their activity. Xylem vessels have no such cells.
• B. The sieve tube opening can be closed by callus, that can be dissolved when not needed.
• C. Sieve tubes have valves that can open and close when needed
• D. Sieve tubes have a large vacuole, no nucleus and a peripheral cytoplasm

Answer 1

The correct option is B

The sieve tube opening can be closed by callus, that can be dissolved when not needed.

Sieve tubes elements can be closed by callus when it is not needed. The callus is dissolved to open the sieve tubes when needed.