Why does secondary growth take place only in dicot roots ?Why not in monocot roots?

Answer 1

secondary growth is, by definition, produced by a vascular cambium. Dicot roots have a this type of cambium, but monocots do not.

Answer 2

In many vascular plants, secondary growth is the result of the activity of the vascular cambium. The latter is a meristem that divides to produce secondary xylem cells on the inside of the meristem (the adaxial side) and secondary phloem cells on the outside (the abaxial side). This growth increases the girth of the plant root or stem, rather than its length, hence the phrase "secondary thickening". As long as the vascular cambium continues to produce new cells, the stem or root will continue to grow in diameter.
Vascular cambium is type of meristem - a tissue consisting of embryonic (incompletely differentiated) cells from which other (and more differentiated) plant tissues originate. Primary meristems are the apical meristems on root tips and shoot tips. Another lateral meristem is the cork cambium, which produces cork, part of the bark.

Vascular cambia are found in dicots and gymnosperms but not monocots, which usually lack secondary growth.

Answer 3

Dicots have open vascular bundle with cambium.Cambium is responsible for secondary growth. Cambium is absent in monocots

Answer 4

there is no taproot,