14 In the book of Genesis, where the lawgiver depicts the paradise planted by God, we are shown that every tree is fair to look upon and good for food; it is also stated that there stands in the midst of the garden a tree of Life and a tree of the knowledge of good and evil; next that the garden is watered by a stream that afterwards divides into four heads. The Prophet Solomon teaches us what this tree of Life is in his exhortation concerning Wisdom: She is a tree of life to all them that lay hold upon her, and lean upon her. This tree then is living; and not only living, but, furthermore, guided by reason; guided by reason, that is, in so far as to yield fruit, and that not casually nor unseasonably, but in its own season.
And this tree is planted beside the rills of water in the domain of the Kingdom of God, that is, of course, in Paradise, and in the place where the stream as it issues forth is divided into four heads. For he does not say, Behind the rills of water, but, Beside the rills of water, at the place where first the heads receive each their flow of waters. This tree is planted in that place whither the Lord, Who is Wisdom, leads the thief who confessed Him to be the Lord, saying: Verily I say unto you, today shall you be with Me in Paradise. And now that we have shown upon prophetic warrant that Wisdom, which is Christ, is called the tree of Life in accordance with the mystery of the coming Incarnation and Passion, we must go on to find support for the strict truth of this interpretation from the Gospels.
The Lord with His own lips compared Himself to a tree when the Jews said that He cast out devils in Beelzebub: Either make the tree good, said He, and its fruit good; or else make the tree corrupt, and its fruit corrupt; for the tree is known by its fruit; because although to cast out devils is an excellent fruit, they said He was Beelzebab, whose fruits are abominable. Nor yet did He hesitate to teach that the power that makes the tree happy resided in His Person, when on the way to the Cross He said: For if they do these things in the green tree, what shall be done in the dry ? Declaring by this image of the green tree that there was nothing in Him that was subject to the dryness of death.
Source: Homilies on the Psalms (New Advent)