Identifying phloem structures in plant stem sections

Steps:

• Observe elongated cells in the longitudinal section of the stem's vascular tissue.
• Look for sieve plates with pores at cell ends, characteristic of phloem transport.
• Rule out thick, lignified walls typical of xylem.
• Confirm match to phloem components involved in sugar conduction.

Why C is correct:

• Phloem sieve tube elements are living, elongated cells stacked end-to-end with sieve plates for sap flow, visible in electron micrographs of stem sections.

Why the others are wrong:

• A. Companion cells are small, nucleated cells aiding sieve tubes, not the main elongated structure.
• B. Casparian strip is a waterproof band in root endodermal cell walls, absent in stems.
• D. Xylem vessel elements are dead, lignified tubes with pitted walls for water transport, lacking sieve plates.

Final answer: C