Jacob continues with the imagery of the allegory, but transforms it into a personal discussion of his people's decisions to follow or not follow God rather than the function of those images in the allegory itself. In no way is this an explanation of the allegory, that is deemed unnecessary, as it was supposed to self-explanatory. This is a conscious alteration of the intent of the allegory into a springboard for a plea to repent. Jacob expresses wonder that they can hear the true way of God, and then choose to ignore it.
In this they are the main tree whose branches are producing evil fruit. they have what they need to turn to God and live, yet are choosing to turn to evil in God's eyes.
Sociological: Jacob does not delineate his audience, but we may assume that this is a general speech to the community, such as the others that have been recorded. When we attempt to visualize early Nephite communities, it is tempting to depict them as a tight community of believers. Jacob's two recorded sermons will not allow that picture. Certainly some of the seeds of Jacob's current religious problem were sown in the days of Nephi, but they have come to bloom during Jacob's ministry.
Returning to the reconstruction of the social pressures on these early Nephites, their rise to some renown and wealth has placed them in the path of other cultures and competing cities. The economic competition has opened the door for practices that are contrary to the will of God, but probably modeled on acceptable behaviors for these other communities. Where Nephi' original rulership certainly combined the religious and the secular, it appears that society is making some tacit separations, and redefining themselves away from the Israelite cultural roots and towards their non-believing neighbors. Jacob sees this process as the branches of the natural tree becoming wild. The allegory of Zenos is made very personal by the events around him as he sees elements of the Israelite religious/cultural core slipping into apostasy before him.