Q.What is the fate of primary xylem in a dicot stem showing extensive secondary growth?

Primary xylem is not destroyed by secondary growth — it persists, largely unaffected, at the centre of the stem, while it is the primary phloem lying outside the vascular cambium that gets crushed as secondary xylem accumulates.

During secondary growth in a dicot stem, the vascular cambium (which arises between primary xylem and primary phloem) cuts off secondary xylem toward the inside and secondary phloem toward the outside. As secondary xylem accumulates year after year, it pushes the phloem tissues progressively outward, and it is the thin-walled, non-lignified primary phloem (and the earliest-formed secondary phloem) that gets crushed under this mechanical pressure and gradually loses its identity.

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