Commentary Purg XXII 136-138

This tree is nourished from above, through its leaves, not from below, through its root system.  Since it is an evergreen, and indeed a mystical representation of a supernatural tree, it does not require nourishment at all.  The water that moistens its branches may thus be symbolic of the water of Life that came to fulfill the function of the Tree of Life in Jesus, who restored to humankind the immortality lost in Eden.