plants often serve as images of the axis mundi. the image of the cosmic tree provides an axis symbol that unites three planes: sky (branches), earth (trunk) and underworld (roots). in some pacific island cultures the banyan tree, of which the bodhi tree is of the sacred fig variety, is the abode of ancestor spirits. in hindu religion, the banyan tree is considered sacred and is called "ashwath vriksha" ("i am banyan tree among trees" – "bhagavad gita"). it represents eternal life because of its seemingly ever-expanding branches. the bodhi tree is also the name given to the tree under which gautama siddhartha, the historical buddha, sat on the night he attained enlightenment. the mesoamerican world tree connects the planes of the underworld and the sky with that of the terrestrial realm. the yggdrasil, or world ash, functions in much the same way in norse mythology; it is the site where odin found enlightenment. other examples include jievaras in lithuanian mythology and thor's oak in the myths of the pre-christian germanic peoples. the tree of life and the tree of knowledge of good and evil in genesis present two aspects of the same image. each is said to stand at the center of the paradise garden from which four rivers flow to nourish the whole world. each tree confers a boon. bamboo, the plant from which asian calligraphy pens are made, represents knowledge and is regularly found on asian college campuses. the christmas tree, which can be traced in its origins back to pre-christian european beliefs, represents an axis mundi. , such as the fly agaric mushroom among the evenks of russia. in china, traditional cosmography sometimes depicts the world center marked with the jian tree (). two more trees are placed at the east and west, corresponding to the points of sunrise and sunset, as described in the "huainanzi".