Jacob 5 - Understanding Zenos's Allegory
"But certain major symbols must be understood in order to understand the allegory".

Oddly enough, the identity of the roots is missing from this Seminary Student Guide.

Grafting = the joining of one group of people to another.

Look at 5:8 - "And behold, saith the Lord of the vineyard, I take away many of these young and tender branches, and I will graft them whithersoever I will."

The 4th branch is believed by some to be the Lehites/Mulekites. They were not grafted into any existing civilization.

Jacob 5:8, 13-14, 19-25 - The Young and Tender Branches of the House of Israel
"So the Lord took branches like the Nephites, like the lost tribes, and like others that the Lord led off that we do not know anything about, to other parts of the earth."

The 4th branch is labelled as being the last.

"Elder Bruce R. McConkie, who was a member of the Quorum of the Twelve Apostles,explained: Those born in this lineage [the lineage of Jacob], having the blood of Israel in their veins and finding it easy to accept the gospel, are said to have believing blood" (Mormon Doctrine, page 81).

Humourous. Gentiles outside the lineage of Jacob have non-believing blood and yet are able to believe the Gospel of Christ.

A variation of this teaching still finds itself present in Doctrines of the Gospel Student Manual - Religion 430-431. Page 57 - C.1 - "This first Comforter or Holy Ghost has no other effect than pure intelligence ... while the effect of the Holy Ghost upon a Gentile, is to purge out the old blood, and make him actually of the seed of Abraham. That man that has none of the blood of Abraham (naturally) must have a new creation by the Holy Ghost. In such a case, there may be more of a powerful effect upon the body, and visible to the eye, than upon an Israelite, while the Israelite at first might be far before the Gentile in pure intelligence" (Smith; Teachings, pages 149-150).

Jacob 5:32-46 - When Did All the Trees become Corrupt?
"The part of the allegory when all the trees bear evil fruit seems to represent the period of the Apostasy, prior to the restoration of the gospel."

Seems to represent the apostasy? In verse 24, there is a mention of the 3rd branch. This branch bears good fruit and it just disappears from the remainder of the parable. Despite the fact that it is not recorded as bearing evil fruit, the Seminary Guide writer mentions The Third Visit as being The Great Apostasy.

For more information, read the analysis of Grant Gardner's interpretation.