The Axis Mundi (or world axis) is analagous to the Rotational Axis of the Earth which is an idealized line running through the middle of the globe to the Pole Star, and has been symbolized as a column or pillar, as a tree, a ladder, and a mound, mountain or pyramid, all of which are seen as forming a connection between the earth and the heavens (the sky). [Note that while a tree may be stripped of it's limbs to fashion a column, at that point it ceases to be a tree, and while some pyramids may have been fashioned with steps leading to the top, a pyramid is not a ladder.]
No matter how it is portrayed, the Axis Mundi is seen as a mediator between the earth and the heavens, and between pairs of opposites in general; just as column is seen as connecting a floor and a ceiling. For instance, when the just man is said to have his feet on the earth and his head in the heavens, the metaphor is of a mediator between the material and spiritual.
It should be pointed out, that the axis mundi is a "vertical metaphor", unlike the notion of an arch, which is seen as mediating between two parallel pillars (side by side), a horizontal metaphor.
In each of these examples, the sense of movement or direction that is being portrayed is from the the earth, matter or the body toward the heavens, mind or the spiritual realm, symbolic of the spirit in mankind seeking to transcend it's animal or material nature. Revelation 3:12 reads, "Him that overcometh will I make a pillar in the Temple of my God and he shall no more go out"; and in Rev.2:7, "To him that overcometh will I give to eat of the Tree of Life, which is in the midst of the paradise of God".
You may recall stories of Menes, the first king of Egypt, unifying the Upper and Lower Kingdoms there. (Note that upper and lower refer to the Nile River which runs to the north, so that the upper kingdom is south of the lower kingdom.) The vulture was a symbol of the upper kingdom, while the cobra was a symbol of the lower one, and we see them depicted on images of pharoahs after Menes, as symbols of a Unified Egypt.
You will note that the dual cobra/vulture symbol is what C.G. Jung calls a symbol of transcendence, one representing two realms and the bridge between the two. In Nietszche's "Zarathustra", the hero descends from the mountain with an eagle and a serpent. The wizard in Disney's "Alladin" movie is accompanied by a staff shaped like a cobra, and a parrot. Each of these is symbolic of the earth and the heavens, between which stands Man, the wizard, the Initiate, etc.
Sometimes the two aspects are united in one form as we see in the winged horses and dragons from Europe and the East, or the feathered-serpent from Meso-america.
A warfare between the Upper and Lower Lands and their kings was generally a part of the history of Egypt, ending in the conquest of the Lower by the Upper and the union of the two under the crown of dual sovereignty. This drama was enacted so often in the history of so many kings of Egypt that even a scholar of the eminence of the late William H. Breasted, in his History of Egypt, expresses his puzzlement over the fact that nearly every Pharaoh of the dynasties had to conquer Lower Egypt afresh and unite the two halves of the country!
Some authors have suggested that Egypt is used in the Bible to symbolize the body, and that the phrase "out of Egypt" symbolizes the ascent on the tree of life, with Israel (the promised land etc) symbolizing the spirit. [Genisis 45:25 "And they went up out of Egypt, and came into the land of Canaan unto Jacob their father."] This adds another layer of meaning to the symbols of the vulture and serpent and the upper and lower kingdoms; indicating the spirtual and material aspects of humanity; with the Nile serving as the axis this time.
The two kingdoms are also represented by plants, the upper kingdom by the lotus and the lower by papyrus, and an image known as "Unity of Egypt" consists of a windpipe stretched so that it looks like the trunk of a tree, indicating the main axis of Egypt (the Nile), along with a small set of lungs, flanked on either side by the lotus and papyrus as symbols for each of the kingdoms.
Z. Stecchinni, in an appendix to Peter Tompkins' "Secret of the Pyramids", recommends that the Tree of Life, symbolic of the unification of opposing forces, "proves to be modeled on the Unity of Egypt" (page 301 "Secrets of the Great Pyramid"). Notice the three pillars and how the plants knotted around the axis form the X at the solar center. Note that the tree mediates both left and right, and up and down.
The opposition of the upper and lower kingdoms is reflected in the mythological conflict between Horus and Set, also symbols of the upper and lower kingdoms, pictured as a falcon (bird) and crocodile (serpent) respectively. The image below reminds us of familair pictures of Michael or St George slaying the dragon, a symbol of overcoming our animal natures.
The Unity of Egypt glyph is also depicted featuring Horus and Set tying the knot around the axis.
Some people utilize the palm tree as the tree of life and axis mundi.