Like the octave, the spiral, the mandala, the fractal, the image of the tree represents a template for the structure of reality, and the emergence of multiplicity (branches, twigs and leaves) out of unitty (the trunk), of the visible (the tree above ground) out of the invisible, (the hidden part of the tree - the roots below ground).
Many ancient cultures incorporate the myth of the World Tree, or Tree of Life, Tree of Knowledge, Tree of Speech, as it is also known.
In Nordic mythology the World Tree or Yggdrasil of the Eddas is the giant ash tree that links and shelters all the worlds. Beneath the three roots the realms of Asgard, Jotunheim, and Niflheim (Gods and giants) are located, and also the Well of Wisdom and the Well of Fate, and the Hvergelmir (Roaring Kettle), the source of many rivers. In the Tongan/Fijian myth of creation there is at the beginning of things the "Tree of Speech" which, like the giant Ash Yggdrasil, this is the gathering place of the gods, and the tree grows by a fountain, the Water of Life. The three worlds are represented by three parts of the world tree - branchs, trunk and roots. The branchs are the home of the Gods, the middle is the earth / land of the giants, and the roots of the tree is the underworld.
The Bhagavad-Gita ( chapter15 verse 1) refers to the Asvattha (Ficus religiosa, popularly known as the Bodhi tree,) or world tree as growing with its roots in the heavens, and its trunk and branches extending downwards to earth. Its leaves are the Vedas (sacred scriptures); its branches the three qualities (gunas), and the lesser branches the sense-objects
The upside down tree (the metaphor also and more generally applying to the large Nyagrodha (Ficus bengalensis), with branches which grow roots and roots which grow as branches) represents the emergence and emanation of the universe out of the Ground of being.
As well as literal symbolism there is also the more metaphoric. In the system of Kabbala (esoteric Judaism, later adopted and modified by the Hermetic magickal tradition as Qabalah) there is the Tree of Life with its ten Sephirot and twenty-two connecting paths.
There are also diagrams that are tree-like, although not specifically represented as trees. For example the Egyptian and Greek Theogenies, or geneologies of the Gods, trace the origin of all the gods and hence all craetion back to an original God or primal principle (e.g. chaos). More abstract and universal are the Indian Samkhyan diagram of tattwas (evolutes), and Neoplatonist Proclus' tabulation of the emanation of principles from the One to the body.
The theme of the Tree of Life also appears more recently (19th century) in the writings of the Darwinist philosopher and scientist Ernst Haeckel, who was the first (I think) to represent living organisms in a phylogentic tree. In the later twentieth century the Haeckelian tree was superceded by the more precise cladistic tree; this new version of the phylogentic Tree of Life can ideally be used to match the entire evolution of life on Earth down to the smallest branches (although in practice this isnt possible because of limitations of the fossil record)
See also Belorussian translation of this page
World Trees - By Hazel Minot - a very informative on-line essay
Five Trees - Five Sacred trees of India
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