Q.Wood is:

Extensive secondary growth crushes the primary xylem, which lies helplessly at the very centre of the mature axis.

Secondary growth in a dicot stem is driven by the vascular cambium, a lateral meristem that forms a continuous ring between the primary xylem and primary phloem. Every growing season, the cambium cuts off new secondary xylem towards the inside and secondary phloem towards the outside. Because secondary xylem accumulates in enormous quantity over years (this is what becomes 'wood'), the vascular cambium ring, along with everything outside it, keeps getting pushed further and further outward.

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Wood is simply the common/everyday name for secondary xylem, the tissue laid down by the vascular cambium towards the inside of the stem during secondary growth.

In woody dicot stems, the vascular cambium (a lateral meristem) produces secondary xylem towards the inside and secondary phloem towards the outside each growing season. Secondary xylem accumulates in large amounts year after year (forming annual/growth rings) because old xylem is not shed, whereas the thin layer of secondary phloem produced each year gets crushed and is eventually replaced by new phloem, so it never accumulates to any significant thickness. This bulk of accumulated secondary xylem, composed of trache …

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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