Anatomy of Flowering Plants

Protoxylem is the first-formed primary xylem; it differentiates while the organ (root or stem) is still growing in length.
Its tracheary elements have lignin laid down only as rings (annular thickenings) or spirals (spiral thickenings), leaving most of the wall thin.
Such walls can be stretched, so these elements can extend as the surrounding tissues elongate, without rupturing immediately.
Metaxylem, with reticulate, scalariform and pitted thickenings that cannot stretch, is formed later, after elongation has stopped.
Hence annular and spiral elements develop in the protoxylem when the root or stem is elongating.
Note: the key in the source PDF marks 'Differentiating'; the standard answer for this question is 'Elongating', which is taken as correct here.