The Buddha it is said, wished to be represented by the Bodhi-tree; not by image or symbol.  At the Kalingabodhi Jataka (J 4.228) Ananda wishes to establish a sacred or, the same, a hallowed (cetiya) substitute for the Buddha by which people might make their offering of wreaths and garlands. The Buddha asks Ananda how many kinds of hallows there are.  Ananda replies there are three kinds: 1) those of the bodily relics; 2) those of association; 3) those prescribed (i.e.,symbolic).  The Buddha then rejects the first and the last, recognizing only association.  So what does association mean?  The Buddha answers, Only a Mahabodhirukkha, i.e., the Great Wisdom tree, that has been associated with a Buddha is fit to be hallow (cetiya), whether the Buddha be still living, or thoroughly-unbound (pari-nirvana) [from the temporal].  It is clear that this tree is no ordinary tree.  The Buddha has something else in mind.
    Perhaps we might think of this hallow tree as the procession of Bodhi illumination (since rukkha, meaning tree, also connotes illumination). This procession would be like an illuminative branching going in all directions.  From another angle, the Mahabodhirukka, in its emanation phase, is active, universal compassion (which is different than human compassion). In this depiction, it is Bodhi illumination revealing itself to its inchoate side, i.e., the sentient beings who are still living under the spell of self-materialization, unable to awaken from the nightmare of linear rebirth into corporeal bodies.  Helping to grasp this point, it is important to remember that in the Buddha legend, the Buddha, after his enlightenment, showed true compassion by teaching beings how to overcome rebirth.  More than this, the Buddha understood that rebirth, by means of desiring painful corporeality, was the fundamental cause of suffering.
    We also need to consider the ascension phase with regard to this tree (i.e., our wish to become enlightened).  But before we do, we need to consider the tree of natural life in which the primordial Mind, being at first, inchoate (avijja), wrestles with its self-creations of which it has no idea (sambodhi) that it is their lord and master.  Its growth ends, hopefully, in flowering (bodha) which is to say, its natural growth ends in the beginning of its liberation from its conflicting self-creations.  This, of course, is the inception of the Bodhisattva path to Buddhahood.  It is the ascension phase; but one which is an inversion, that is, it goes from the flower back down to the root.  Thus, by an excruciating process of trial and error, our flowered Mind (Bodhicitta) eventually must return to its primordial root of the tree of natural life through remembrance.  In sum, working backwards from the flower to the branches then past the trunk to the root, is properly speaking, the path of the Bodhisattva which we earlier termed the ascension phase.  It culminates in enlightenment where the Bodhisattva sits at the root of the Mahabodhirukkha, i.e., the great Bodhi illuminating tree.
    At the base of the tree, the Bodhisattva (who is awakened) begins to generate the illuminating tree (Mahabodhirukkha) which is superessential compassion (the phase of emanation).  It shines downwards, like a sun, into the multiplex branches of phenomena (i.e., the Five Aggregates).  As it were, the tree brings illumination into the dark underworld of beings.  By doing so, it breaks the spell of matter which has a hold on the sentient beings, showing the most worthy of beings this immortal light which ends matter’s rule.  And so, another cycle commences.  Those beholding this divine tree of light, glowing and flowering within their hearts, begin their careers as fledgling Bodhisattvas who will one day, themselves, become Buddhas sitting at the base of the Mahabodhirukkha only to repeat the salvific emanation cycle again.
    To sum this up, we have essentially three trees.  The first is the tree of nature.  The second is the tree of inverted ascension which the Bodhisattva takes in order to get to the root or base of the first tree. The third tree is the enlightenment tree, which like the sun, radiates the salvific light through all branches unto the last. On the issue of this salvific light, a few ideas need to be laid out lest we trivialize the meaning of light in this context.  This light of which we are addressing, first of all, is deathless.  So that by beholding it one comes to understand that it is the power of animation which is the immortal substance (S., amritadhatu).  When one becomes a witness to this light they then commence to renounce the kingdom of matter, for renunciation means to turn to the source of all.  Most importantly, this light has an end.  This end is the dynamic power of illumination, which is a sheer productivity.  This is the root (mula) of Buddhahood, or the same, the stage of the non-reversing Bodhisattva which some scholars hold begins with the eighth Bodhisattva bhumi (ground) of the ten bhumis (Dashabhumi).  It should be pointed out that saving sentient beings is synonymous with illuminating others, or the same, awakening the sleeping mind which has been bewitched by matter symbolized by Mara the Evil One, with whom the Buddha fights while sitting at the base of the Mahabodhrukkha.