Compare7 marksChallenge
A student examines a stained transverse section of a stem from a herbaceous dicotyledonous plant under a light microscope. They identify vascular bundles containing xylem and phloem. The student notes that xylem vessel elements and phloem sieve tube elements have distinct structural features, and that each sieve tube element is closely associated with a companion cell.
Compare the structure of xylem vessel elements and phloem sieve tube elements, and explain why sieve tube elements require companion cells for their function.