'A shrub' or 'plant,' as in Genesis 21:15, denotes perception, but so little that it is hardly anything, in same way as 'trees,' but to a lesser degree. So 'to be cast under one of the shrubs,' signifies being destroyed regarding truth and good, to the point of desperation.

(References: Arcana Coelestia 2682 [1-3])

Jerusalem, on Mount Zion, signifies the doctrine of love to the Lord, and how it governs your life. Jerusalem first comes to our attention in 2 Samuel 5, when King David takes the city from the Jebusites and makes it his capital. In the next chapter he brings the Ark of the Covenant there, and later it is where Solomon builds the temple, and his own palace. From then on Jerusalem is the center of worship of the Israelitish church. It is the place where the Lord was presented in the temple as a baby, where He tarried to talk to the priests at age twelve, where He cleansed the temple, had the last supper, was crucified and then rose. It is a central place in both the old and new Testaments. The city was built on Mount Zion, the highest point of the mountains of Judea. A city, in the Word, represents doctrine, the organized knowledge of the truths of the church. Mountains represent love of the Lord and the consequent worship. If you put those things together, Jerusalem on Mount Zion signifies the doctrine of love to the Lord, and how it governs your life. This is why David was led to make Jerusalem the most important city of the land, and why all worship was conducted there. And this is also why Jeroboam was condemned for introducing idol worship in Samaria. In the Book of Revelation, John's vision of the city New Jerusalem descending from God is a prophecy of a new dispensation of doctrine coming from the Lord.

(References: Arcana Coelestia 4539, 8938; The Apocalypse Explained 365 [35-38])