Q.What is the fate of primary xylem in a dicot root showing extensive secondary growth?
a. It is retained in the center of the axis
b. It gets crushed
c. May or may not get crushed
d. It gets surrounded by primary phloem

Step 1. Recall the position of primary xylem in a root. In the primary structure of a dicot root, the xylem is exarch and occupies the very centre of the stele, typically in a tetrarch (four-point) star-shaped pattern, with primary phloem located in separate patches around it.

Step 2. Recall what happens as the vascular cambium becomes active. Once the cambium forms (from conjunctive tissue and pericycle) and begins producing secondary xylem inward and secondary phloem outward, the new secondary tissue accumulates around the existing primary tissue. Because the cambium forms outside the primary xylem, the growing secondary xylem surrounds the primary xylem rather than growing through or displacing it, so the primary xylem is not put under the same crushing pressure that the primary phloem experiences.

Step 3. Contrast with primary phloem. Primary phloem, positioned outside the developing cambium, does get progressively crushed as secondary phloem accumulates between it and the epidermis/periderm - it is squeezed against the expanding tissue with nowhere to go, and is eventually obliterated. …

Recall the exarch, centrally-placed position of primary xylem in a root and contrast its fate with tha …

• Mixing up the fates of primary xylem (retained centrally) and primary phloem (crushed) - it is easy to assume both primary tissues are crushed equally. …