St. Ambrose has something to say about St. Bartholomew as well. In his work De Isaac vel anima 8.73 he makes use of the Canticle of Canticles and brings in Nathaniel with his fig tree. Remember, Nathaniel and Bartholomew are often associated.
Here is Ambrose:
Because there is now a union of love, the bridegroom caresses her and says, "Under the apple tree I raised you up. There your mother brought you forth, there she brought you forth who bore you." (Cant. 8.5 LXX) Good is the soul that rests under the tree that is fruitful and especially the tree of a good fragrance. For if good Nathanael, in whom there was not any guile, was seen under a fig tree, surely the soul is good that was raised up by her bridegroom under an apple tree. It is a greater thing to be raised up than to be seen, and greater still to be raised up by the bridegroom. For although Nathanael was seen under a tree, still his soul was not a bride, for he came to Christ secretly because he was afraid of the Jews. She was not beautiful as the moon, choice as the sun, (cf. Cant. 6.10) for she was in the shadow, whereas the bride is married in the day and declares it openly. And so the one was under an apple tree, the other under a fig tree, because the one spread the fragrance of her profession of belief over a wider area, the other possessed the sweetness of purity and blamelessness but did not possess fragrance of spirit.
Ambrose sees Nathanael in a different way in another work, however, namely his Explanation of the Gospel of Luke 8.90.