This part of the allegory is concerned with the gathering of Israel. The allegory spoke of Old World branches that had been scattered, at least including the lost ten tribes. Those are to be gathered in and brought back to the house of Israel. The New World branches are to be gathered. They too are to be reunited. The allegory does not specify locations, and those natural branches in the poor soil are not specifically mentioned, although it is certain that they too are part of the gathering.
In verse 55 it can be a little confusing because it says that they “took from the natural tree which had become wild, and grafted in unto the natural trees, which also had become wild.” The original, or the mother tree, is one of the trees which had become wild. The other branches that became wild were the young and tender shoots that were originally broken off and placed in far regions. The botanical process is to cross-graft branches among the trees. The spiritual process is to reunite scattered Israel through the restoration of the gospel.