Why is primary xylme in root exarch and in stem endarch ? Expain

Explain why: Primary xylem in root is exarch and in stem it is endarch.

primary stem is

primary stem is

Two cross sections of stem and root appear simple, when viewed by naked eye. But under microscope they can be differentiated by (a) Exarch condition of root and stem (b) Endarch condition of stem and root (c) Endarch condition of root and exarch condition of stem (d) Endarch condition of stem and exarch condition of root

In stems, the primary xylem is called endarch, because -

In roots, the primary xylem is called exarch. because -

In roots, the primary xylem is called exarch. because -

Which is the primary function of stem ?