Q.Grafting is successful in dicot plants but not in monocot plants because the dicot plants have :

The vascular cambium is the lateral meristem responsible for producing a dicot or gymnosperm stem or root's secondary vascular tissues - secondary xylem (wood) on its inner face and secondary phloem (bast) on its outer face - and it is one of the two lateral meristems (alongside the cork cambium) that together carry out secondary growth. Its origin differs between organs: in a stem, part of the eventual cambial ring - the intrafascicular (fascicular) cambium - is already present inside each vascular bundle from early development, believed to originate from the procambium, and is therefore primary in nature from the start; the gaps between bundles are closed by interfascicular cambium, which forms later when parenchyma cells of the medullary rays in line with the intrafascicular cambium become meristematic on their own, making it secondary in nature from the beginning. These two components join on both sides to form one continuous vascular cambial ring. In a root, by contrast, the vascular cambium is completely secondary in origin, forming from a combination of conjunctive tissue below the phloem strands and part of the pericycle above the protoxylem, initially as a wavy ring that only later becomes circular. …