There is no distinction between tunica and corpus layers in the stem apices of

$A$ : According to Clowes, there are three histogens in monocot root.

$R$ : In dicot roots, innermost groups of initials form root cap.

Sieve tubes are better suited for translocation, because

Amphiphloic $(bicollateral)$ condition of stele means that

Initiation of lateral roots and vascular cambium during the secondary growth takes place in these cells.