The servant and the master return from planting the "young and tender" branches in various places and witness the progress of the original tree. The passage of time here is once again indicative of the patient process of the Lord with his children.
In this part of the allegory, the grafting of the wild branches has had the desired result, the added vigor of the wild branches has invigorated the tree, and there is good fruit again.
Symbolic: The fruit of a tree is a frequent metaphor which we have seen in Lehi's dream, and which flows through several New Testament examples. As the ultimate product of the tree, it is the fruit which is the reason for the planting. In that context, the good fruit is the ability of the people represented by the branches to fulfill their God-desired destiny. Through the good tree, they become good fruit, righteous souls for God.
Botanical: The specifics of this verse require some scientific explanation. While there may be some improvement in the fruit, a complete change is not genetically possible. As Hess, et al note:
"…a wild olive graft does not genetically become tame. Each cell of any branch will remain genetically the same as the parent tree from which it was cut. Cultural practices involving increased nutrition, proper pruning, irrigation, and so forth, will not cause wild fruit to attain the same size and desirable characteristics as tame fruit, but some improvement might be shown.
On one occasion Zenos states that the wild branches began to produce good fruit. But the Lord of the vineyard attributes this solely to the strength of the roots, not to any constitutional change: "Behold, the branches of the wild tree have taken hold of the moisture of the root thereof, that the root thereof hath brought forth much strength; and because of the much strength of the root thereof, the wild branches have brought forth tame fruit" (Jacob 5:18). Later the Lord will remember that because of the strength of the roots, "they have hitherto brought forth, from the wild branches, good fruit" (Jacob 5:36). Not only do the servant and Lord of the vineyard seem somewhat surprised that the wild branches have borne fruit that is "like unto the natural fruit" (Jacob 5:17), but this singular event did not last long. In time the wild branches completely overran the roots and the tree became worthless (Jacob 5:37)." (Hess, Wilford M., Daniel J. Fairbanks, John W. Welch, and Jonathan K. Driggs "Botanical Aspects of Olive Culture Relevant to Jacob 5" In: The Allegory of the Olive Tree. FARMS, Provo. p. 511).
The allegory is more interested in the symbolic aspects of the story than the botanical, but the botanical aspects are not impossible, but rather used with some judicious poetic license. As has already been noted, allegories teach because they can relate one set of concepts to another more familiar set, but an absolute point by point contiguity is not requisite.