In the world's centre is a tree of life (the "yaxche" 'ceiba') that serves as a means of communication between the various spheres. In Palenque, the tree of life is a maize tree, just as the central world tree in the Borgia Codex; a curving bicephalic serpent hovers around it, which some believe to embody the ecliptic. The king was probably identified with the tree of the centre and is usually shown to carry the bicephalic serpent as a ceremonial bar. Besides worshipping a central maize tree, the king commonly sits or stands on a mountain containing the maize, perhaps as a guardian of the kingdom's maize supplies.